In Europe we use a lot of diesel-hydraulic locomotives. Let me explain for example a switcher locomotive (Croatian series 2132) how it changes gear ratios. The engine drives the transmission which has two identical torque converters but each torque converter is linked to it's own pair of gears. The gear ratios are different. When the locomotive is given the command to start moving the first torque converter with the lower gear ratio is being filled with oil. As the locomotive accelerates and depending how much throttle you have given the electronics tell the transmission to drain out the oil from the first torque converter and start filling the oil to the second torque converter which is coupled to the pair of gears which give a higher output shaft speed and the locomotives accelerates to higher speeds. The concept is pretty simple, like an automatic car but instead of shifting planetary gearsets you have multiple torque converters and each of them has it's own gears with a different gear ratio. Only one converter is filled with oil at any given time. For a short amount of time (about few seconds), when the transition from one to another torque converter is being made, two converters are filled with oil and this ensures a smooth shift with no loss in traction and no jerking.
Some early automatic transmissions were all torque converters and no gears, like the Buick Dynaflow and the Chevrolet Turboglide. These existed in the 1950s and '60s and were gone by 1965.
I've never had a clear understanding of this type of drive, and the madness that went into creating it, but your video has given me a clear understanding. Thank you. I was a kid when the SP and Rio Grande locomotives were tested and I was like, what? All these years later, now I know.
Literally said on the previous video that I don’t understand the difference between mechanical and hydraulic drives and here’s the answer merely a click away. Brilliant explanations 👍
The multiple torque converters are in PARALLEL, not in series. Each is powered by the prime mover, but outputs to a different ratio geartrain that leads to a single output shaft. The torque converters are sequentially drained to achieve gear ratio changes at the drive wheels. They are not locked up.
The biodiesel engines of the Mt Washington Cog Railway are of the diesel-hydraulic type. They selected that method over diesel-electric as it would have less weight .
In Britain nowadays the only diesel-hydraulics are diesel multiple units which switch into direct drive for the top 'gear'. Back in the 1960's the reason some DH locos were built in Britain was because they were about 15 tons lighter than an equivalent DE loco. It's a long story as to whether they were a success or not. Virtually all of the many types of diesel loco Britain started with, both DH and DE were problematic. Some classes got ditched quickly but the rest did get sorted out eventually. By the 1970's most were running with a fairly similar miles-per-falure rate. But it was clearly inefficient having so many types and some had to go. Against the DH locos was there were less of them, they could only do 90mph (a bogie problem) and they could not be adapted to provide electric train heating (hotel power). Also DE loco design was evolving quicker globally. Anyways, there are still (I think?) 11 of the big diesel-hydraulics preserved and running on heritage railways, plus a handfull of shunters. The loco seen at 2m00 is one of them but out on the main line to check it's OK to run on the network. Hopefully it'll be back soon taking enthusiast around the country at 90mph.
I was watching your video about diesel hydraulic locomotives when at 3:29 by way of illustration there was a short section showing a Class 14 "Teddy Bear" locomotive rolling into Ramsbottom station on my local heritage line The East Lancashire Railway. Only the internet provides these sorts of connections. I am really enjoying your videos.
I stumbled over your channel randomly and started by watching your unusual and unique captures rail fanning, I seen your videos explaining the mechanics of how things work and I have to say, you are very well spoken, very clearly explain whatever it is your talking about, and are very easy to follow along. Really glad I found your channel and I hope it grows into something much larger than what it currently is
That was a commercial failure however. The Maxima has just two speeds. First speed uses a torque converter, second speed uses a fluid coupling driving through the same gear. The coupling slips less than the converter so it's effectively a higher gear. If you had a third speed it would use a second fluid coupling driving through a higher gear.
With automatic transmissions in cars, reverse gear is achieved by having the sun as the input, ring as the output and the planetary carrier locked in place.
No, but that’s not a bad idea. I think hydrogen powered/ fuel cell locomotives would be a cool topic. I’ll have to give it a while though, because I’ve regularly been posting “how stuff works” videos for the past 6 weeks.
Is there any diesel-hydraulic locomotives that use individual hydraulic motors on the axles and the engine is just to run a hydraulic pump? kind of like diesel-electric but hydraulic
I don’t know, I didn’t come across any locomotives like that during my research, but I wouldn’t be surprised if a system like that has been been experimented with, or implemented on smaller rail vehicles (like MOW equipment).
When I was an apprentice our mentor said years back a hydrostatic with a VD pump locomotive had been trialled once. But would always blow hoses and generally never worked right. The technology wasn’t there. Maybe now it could be made to work. But while it would be super efficient compared to TCs etc. there’s no reason to believe it would be better than diesel electric.
Diesel hydraulic locos use oil in the transmission to fill the relavent torque convertor to engage the gear req. To change to another gear the first convertor is drained of its oil then a second convertor is filled with the same oil to drive through a higher ratio to give higher speeds.
I’ve read of some of the torque converter schemes in old books, and would not be surprised if actual automatic transmissions have been tried at some time or other, but I’d have guessed that the overwhelming majority of diesel hydraulic locomotives employ hydrostatic drives.
The UK hindered itself with Diesel Hydraulic locos for a while - which they got rid of - one of the designs was the Blue one with small yellow panel at the front. Got rid because they were non-standard. Some of the Passenger Units are using D-H nowadays.
5:03 What is the big yellow thing the locomotive has on the front, just above the coupler? I know it uses a buffer and screw coupler, but they do not normally look that way.
First off, Nice video. Small correction; @ 6:53. I think you stated it backwards. I always took convention to mean output vs input?, so saying 'increased torque' means more torque at the output, to me: ie. more speed, less torque on the input side (input from the engine), and MORE torque and LESS speed at the output. If that's how you mean it, then you said it backwards: it would be 'driving ring, driven planetary, (stationary sun)' is more speed, at the expense of torque; and 'driving sun, driven planetary, (fixed ring)' is a LARGE increase in torque, less speed... [both have an output speed less than the input...] Do I have that right? Mr K L
1:58 I wonder what lead to a modern class 66 on tanks being rescued by a Western? It looks rather strange considering how soon hydraulics were abandoned in the UK.
fluid couplings and or torque converters have a problem of cavitation if you remove too much fluid. Bubbles form on the meal destroying the metal when they burst too much pressure SO what happens if you pressurize the fluid drive or torque coupling body??? the bubbles cannot burst because the pressure in the converter body is too high so now the fluid can get even thinner making the reduction, speed lower and torque greater.........
In India today there is little to none Broad Gauge Diesel - Hydraulic locomotives in Operation... 90% of our Broad Gauge rail routes are electrified and the rest works are done by ALCos and EMD's Diesel - Electric Locos... There used to be a loco called WDM 3 ( not to be confused with WDM 3A or WDM 3D ) but they were not successful... For Meter Gauge or Narrow Gauge there were some but now their network compromised less than 5% of our network... For Meter Gauge, the remaining network use YDM 4 Diesel Electric Locos only... For Industrial railways there might be some...
i often commute by train in italy, the trains on that railway are always the same model diesel-multiple-unit model, i always assumed they were diesel-electric, as the same trainset is also available as an EMU, however i looked into it and apparently it's diesel hydrodynamic with no gears. a different nearby railway did have some diesel trains with gears, i don't know what kind but now the line is getting electrified so they wont be use anymore
The real reason diesel-hydraulics are used instead of diesel-electric is because some transmission company wanted to make money. There is no real technical benefit. The peak efficiency may be higher, but the average efficiency is not.
@@marioxerxescastelancastro8019 The main advantage compared to older DC locos is that you can stall the output (axles with the brakes and not kill the traction motors) so driving REALLY slow is possible without needing modern computer tech and VFDs. modern Hydraulics are often bought by companies, cause they do not have any large drivetrain computers, so theytend to be fixable by the small workshop and a manual instead of a IT-degree. Older hydraulics are very common in worktrain usage and switching, cause they do exactly what the operator wants them to do, without any computer arguing if it really is a good idea, a old 1200HP hydraulic can do stuff a modern AC loco can't do, cause the computer doesn't like it.
At least back in the 1950s and 1960s Diesel hydraulic locos had a significantly better power to weight ratio, which mattered a lot for 4 axle designs. In those days coaching stock was usually still steam heated requiribg a heating boiler on the locos. Later on this turned the tide for diesel-electric
@@marioxerxescastelancastro8019don't know what is used where you are, but while diesel hydraulic locos were never that common diesel hydraulic multiple units are and in some cases they replaced diesel electric units as they are I think cheaper and lighter. More recently conventional automatic gearboxes have been used.
Here's a Romanian built diesel-hydraulic still in use in Vietnam. I haven't been inside it yet, but it has one drive motor in the center, with a drive shaft to each wheel-set. ua-cam.com/video/EGiDI_8lIkw/v-deo.html
Speaking of the Espees Diesel hydraulics, it should be considered nothing short of a miracle that 9010 has made it to where it is today, considering how the unit was nothing more than a bare shell when she arrived at Niles Canyon
I think you err in regard to the use of planetary gears to increase speed. You have to fix the sun gear and drive the planet carrier. If as you said you fix the ring gear and drive the sun gear, it's still a reduction.
Yknow, I just thought of something, if there have been trains with a hydraulic transmission, I wonder if cars, trucks or even ships ever had hydraulic transmissions 🤔
Well, the automatic transmissions in vehicles are shifted through a combination of hydraulics and electric solenoids. As for the torque converters being used as a transmission, I’ve never heard of a vehicle with such a thing. Now on a boat, torque converter transmissions are more feasible, but I still don’t know of any examples.
The Konegsiegg Regera got sort of kind of a hydrolic drive. While its a hybrid, the maker claim that it in theory could run on hydric drive only (while it would have not reverse gear). If you want to be really literal, all ships have hydrolic drive... due to.. water being a liquid. I have seen boats with hydrolic drives, but they where not used for gearing. They was used so the outbord propeller linked to the onboard engine with no saft, making it easier to integrate.
It could be, I suppose, but most of the passenger trains where I am are diesel hydraulic and it doesn't seem to be a problem. In any case given that the transmission is likely to be in the middle any oil that escapes will probably fall between rather than on the rails.
I was looking throw different type of electric locomotive drive.... and one might suspect, how many different types can there be.... a lot.. REALLY a LOT. I found like 15 different ways just from a breef look. There is probobly loads more. And i don´t know if there going to be more parts of this series, but there is also diesel pneumatic. About electric, as far as i can find there are 5 main categories. AC-AC AC-DC (yes, like the metal group) AC-DC-AC DC-DC DC-AC For something like AC-DC-AC that is the most common today, there is at least two different subtypes, syncronic or asynchronous motor
@@russellgxy2905 typically its a 3 pole H bridge with transistors or thyristors. If made with transistors they can be feed backwards to be used as a rectifier. This is actually more efficent then diods. So most modern locos have H bridges on both side. Both to the motors as well as the power line. This way they can feed the power back put when breaking.
In Europe we use a lot of diesel-hydraulic locomotives. Let me explain for example a switcher locomotive (Croatian series 2132) how it changes gear ratios.
The engine drives the transmission which has two identical torque converters but each torque converter is linked to it's own pair of gears. The gear ratios are different. When the locomotive is given the command to start moving the first torque converter with the lower gear ratio is being filled with oil. As the locomotive accelerates and depending how much throttle you have given the electronics tell the transmission to drain out the oil from the first torque converter and start filling the oil to the second torque converter which is coupled to the pair of gears which give a higher output shaft speed and the locomotives accelerates to higher speeds.
The concept is pretty simple, like an automatic car but instead of shifting planetary gearsets you have multiple torque converters and each of them has it's own gears with a different gear ratio. Only one converter is filled with oil at any given time. For a short amount of time (about few seconds), when the transition from one to another torque converter is being made, two converters are filled with oil and this ensures a smooth shift with no loss in traction and no jerking.
So the difference between a diesel mechanical and a diesel hydraulic is the "transmission"?
Some early automatic transmissions were all torque converters and no gears, like the Buick Dynaflow and the Chevrolet Turboglide. These existed in the 1950s and '60s and were gone by 1965.
I've never had a clear understanding of this type of drive, and the madness that went into creating it, but your video has given me a clear understanding. Thank you. I was a kid when the SP and Rio Grande locomotives were tested and I was like, what? All these years later, now I know.
Literally said on the previous video that I don’t understand the difference between mechanical and hydraulic drives and here’s the answer merely a click away. Brilliant explanations 👍
Lol, glad you liked the video!
@@Southern_Plains_Railfan Very much. Thanks so much for these.
The multiple torque converters are in PARALLEL, not in series. Each is powered by the prime mover, but outputs to a different ratio geartrain that leads to a single output shaft. The torque converters are sequentially drained to achieve gear ratio changes at the drive wheels. They are not locked up.
I don't think he meant series in that manner. He probably used it to refer that there is more than one.
Why can't they be in series? Sounds like smoother gearing to me 🤔
It’s really Interesting how the Various Locomotives are Powered and it’s something most people ever give much thought to. Thanks for Explaining. 👍
You’re welcome! Yes, the way anything works is usually disregarded by most people.
The biodiesel engines of the Mt Washington Cog Railway are of the diesel-hydraulic type. They selected that method over diesel-electric as it would have less weight .
In Britain nowadays the only diesel-hydraulics are diesel multiple units which switch into direct drive for the top 'gear'. Back in the 1960's the reason some DH locos were built in Britain was because they were about 15 tons lighter than an equivalent DE loco. It's a long story as to whether they were a success or not. Virtually all of the many types of diesel loco Britain started with, both DH and DE were problematic. Some classes got ditched quickly but the rest did get sorted out eventually. By the 1970's most were running with a fairly similar miles-per-falure rate. But it was clearly inefficient having so many types and some had to go. Against the DH locos was there were less of them, they could only do 90mph (a bogie problem) and they could not be adapted to provide electric train heating (hotel power). Also DE loco design was evolving quicker globally. Anyways, there are still (I think?) 11 of the big diesel-hydraulics preserved and running on heritage railways, plus a handfull of shunters. The loco seen at 2m00 is one of them but out on the main line to check it's OK to run on the network. Hopefully it'll be back soon taking enthusiast around the country at 90mph.
I was watching your video about diesel hydraulic locomotives when at 3:29 by way of illustration there was a short section showing a Class 14 "Teddy Bear" locomotive rolling into Ramsbottom station on my local heritage line The East Lancashire Railway. Only the internet provides these sorts of connections. I am really enjoying your videos.
I stumbled over your channel randomly and started by watching your unusual and unique captures rail fanning, I seen your videos explaining the mechanics of how things work and I have to say, you are very well spoken, very clearly explain whatever it is your talking about, and are very easy to follow along. Really glad I found your channel and I hope it grows into something much larger than what it currently is
Thank you so much!
Love the CONRAIL picture great video ❤
Thank you!
NP
The most powerful european DHC or Diesel Hidraulic locomotive is the Voith Maxima. 5000HP. And is a mainline locomotive. Grate video. Grate work.
That’s cool! Thank you!
That was a commercial failure however. The Maxima has just two speeds. First speed uses a torque converter, second speed uses a fluid coupling driving through the same gear. The coupling slips less than the converter so it's effectively a higher gear.
If you had a third speed it would use a second fluid coupling driving through a higher gear.
With automatic transmissions in cars, reverse gear is achieved by having the sun as the input, ring as the output and the planetary carrier locked in place.
It was a Heavenly Video for a Railfan like me . Thanks for making such informative video. watched the whole series of yours. It was Fabulous !!!
A western pulling a class 66 and cement wagons I’ve seen it all
The western has to be one of the most beautiful locomotives ever!
Hear, hear!
For those who aren’t aware, hydrostatic drives are also used in lawnmowers and other small tractors
Yeah. My John Deere has it
Any plans for doing vids about electrical driven trains? (IE power line trains, battery trains, fuel cell trains)
No, but that’s not a bad idea. I think hydrogen powered/ fuel cell locomotives would be a cool topic. I’ll have to give it a while though, because I’ve regularly been posting “how stuff works” videos for the past 6 weeks.
Is there any diesel-hydraulic locomotives that use individual hydraulic motors on the axles and the engine is just to run a hydraulic pump?
kind of like diesel-electric but hydraulic
I don’t know, I didn’t come across any locomotives like that during my research, but I wouldn’t be surprised if a system like that has been been experimented with, or implemented on smaller rail vehicles (like MOW equipment).
could have been or some light shunters maybe, or maybe some form of hydrostatic drive setup
That’s how I thought they all ran, a diesel electric type with hydraulics instead of electrics.
When I was an apprentice our mentor said years back a hydrostatic with a VD pump locomotive had been trialled once. But would always blow hoses and generally never worked right. The technology wasn’t there. Maybe now it could be made to work. But while it would be super efficient compared to TCs etc. there’s no reason to believe it would be better than diesel electric.
Wow, what an awesome video, very well explained!
Awesome information.
Thank you!
Diesel hydraulic locos use oil in the transmission to fill the relavent torque convertor to engage the gear req. To change to another gear the first convertor is drained of its oil then a second convertor is filled with the same oil to drive through a higher ratio to give higher speeds.
I’ve read of some of the torque converter schemes in old books, and would not be surprised if actual automatic transmissions have been tried at some time or other, but I’d have guessed that the overwhelming majority of diesel hydraulic locomotives employ hydrostatic drives.
Very informative, thank you.
You’re welcome!
The UK hindered itself with Diesel Hydraulic locos for a while - which they got rid of - one of the designs was the Blue one with small yellow panel at the front. Got rid because they were non-standard.
Some of the Passenger Units are using D-H nowadays.
Very good video, CLEARLY explained! New Zealand Locomotives are diesel electric.
Thank you very much!
@@Southern_Plains_Railfan we do have diesel/mechanical smaller but the main ones are diesel/electric.
A lot of multiple unit unit passenger trains trains use diesel hydraulic transmission in the UK and in many parts of Europe.
5:03 What is the big yellow thing the locomotive has on the front, just above the coupler? I know it uses a buffer and screw coupler, but they do not normally look that way.
That is an automatic shunting coupling
Do a whole separate video on the BR class 55 or the Deltic as the engine it uses is quite good
First off, Nice video.
Small correction; @ 6:53. I think you stated it backwards. I always took convention to mean output vs input?, so saying 'increased torque' means more torque at the output, to me: ie. more speed, less torque on the input side (input from the engine), and MORE torque and LESS speed at the output. If that's how you mean it, then you said it backwards: it would be 'driving ring, driven planetary, (stationary sun)' is more speed, at the expense of torque; and 'driving sun, driven planetary, (fixed ring)' is a LARGE increase in torque, less speed...
[both have an output speed less than the input...]
Do I have that right?
Mr K L
1:58 I wonder what lead to a modern class 66 on tanks being rescued by a Western? It looks rather strange considering how soon hydraulics were abandoned in the UK.
That was a haulage test for the Class 52 so it could run mainline railtours
@@Schmalfonzo Oh, that makes sense. It looks quite unusual hauling a 66 with a load of tanks.
Don’t get me to lying! Lol
@@Southern_Plains_Railfan I don't know what you mean.
fluid couplings and or torque converters have a problem of cavitation if you remove too much fluid. Bubbles form on the meal destroying the metal when they burst too much pressure SO what happens if you pressurize the fluid drive or torque coupling body??? the bubbles cannot burst because the pressure in the converter body is too high so now the fluid can get even thinner making the reduction, speed lower and torque greater.........
The U.S. KM and ALCO hydraulics all had 2 engines. SP KM 9010 is the only one saved and not scrapped.
Note: Inka (Indonesia) make a Diesel Hydraulic CC300 (CC = Coco bogie 300= Diesel Hydraulic)
Great stuff, I am enjoying this series! (Dave).
Thank you!
@@Southern_Plains_Railfan Your welcome.
In India today there is little to none Broad Gauge Diesel - Hydraulic locomotives in Operation... 90% of our Broad Gauge rail routes are electrified and the rest works are done by ALCos and EMD's Diesel - Electric Locos... There used to be a loco called WDM 3 ( not to be confused with WDM 3A or WDM 3D ) but they were not successful... For Meter Gauge or Narrow Gauge there were some but now their network compromised less than 5% of our network... For Meter Gauge, the remaining network use YDM 4 Diesel Electric Locos only... For Industrial railways there might be some...
i often commute by train in italy, the trains on that railway are always the same model diesel-multiple-unit model, i always assumed they were diesel-electric, as the same trainset is also available as an EMU, however i looked into it and apparently it's diesel hydrodynamic with no gears.
a different nearby railway did have some diesel trains with gears, i don't know what kind but now the line is getting electrified so they wont be use anymore
Can you do a set of videos on which diesel hydraulic designs work best in different applications that diesel hydraulics are used for?
The real reason diesel-hydraulics are used instead of diesel-electric is because some transmission company wanted to make money. There is no real technical benefit. The peak efficiency may be higher, but the average efficiency is not.
@@marioxerxescastelancastro8019 The main advantage compared to older DC locos is that you can stall the output (axles with the brakes and not kill the traction motors) so driving REALLY slow is possible without needing modern computer tech and VFDs. modern Hydraulics are often bought by companies, cause they do not have any large drivetrain computers, so theytend to be fixable by the small workshop and a manual instead of a IT-degree. Older hydraulics are very common in worktrain usage and switching, cause they do exactly what the operator wants them to do, without any computer arguing if it really is a good idea, a old 1200HP hydraulic can do stuff a modern AC loco can't do, cause the computer doesn't like it.
@@FabiaLp I see. Informative answer.
At least back in the 1950s and 1960s Diesel hydraulic locos had a significantly better power to weight ratio, which mattered a lot for 4 axle designs. In those days coaching stock was usually still steam heated requiribg a heating boiler on the locos. Later on this turned the tide for diesel-electric
@@marioxerxescastelancastro8019don't know what is used where you are, but while diesel hydraulic locos were never that common diesel hydraulic multiple units are and in some cases they replaced diesel electric units as they are I think cheaper and lighter.
More recently conventional automatic gearboxes have been used.
Here's a Romanian built diesel-hydraulic still in use in Vietnam. I haven't been inside it yet, but it has one drive motor in the center, with a drive shaft to each wheel-set. ua-cam.com/video/EGiDI_8lIkw/v-deo.html
Unbelievable
Speaking of the Espees Diesel hydraulics, it should be considered nothing short of a miracle that 9010 has made it to where it is today, considering how the unit was nothing more than a bare shell when she arrived at Niles Canyon
I think you err in regard to the use of planetary gears to increase speed. You have to fix the sun gear and drive the planet carrier. If as you said you fix the ring gear and drive the sun gear, it's still a reduction.
Got around to seeing this. Sorry man, you're very confused about how they actually work
What is that massive beast of a transmission at 6:48? 😂
Looks like a truck transmission to me
Looks like a Mercedes hybrid 9-speed unit. Electric motor at the front, then a torque converter, followed by the planetary geartrain.
Yknow, I just thought of something, if there have been trains with a hydraulic transmission, I wonder if cars, trucks or even ships ever had hydraulic transmissions 🤔
Well, the automatic transmissions in vehicles are shifted through a combination of hydraulics and electric solenoids. As for the torque converters being used as a transmission, I’ve never heard of a vehicle with such a thing. Now on a boat, torque converter transmissions are more feasible, but I still don’t know of any examples.
@@Southern_Plains_Railfan ah ok I see
The Konegsiegg Regera got sort of kind of a hydrolic drive. While its a hybrid, the maker claim that it in theory could run on hydric drive only (while it would have not reverse gear).
If you want to be really literal, all ships have hydrolic drive... due to.. water being a liquid.
I have seen boats with hydrolic drives, but they where not used for gearing. They was used so the outbord propeller linked to the onboard engine with no saft, making it easier to integrate.
@@matsv201 well a shaft is a mechanical transmission
@@jadenspires1891 Well.. hydraulic drive can be used for both gearing as well as speed conversion
What is a vidiah?
daysul logomotyves lol
This does not cover hydraulic, only hydrostatic
The diesel hydraulic locomotive is a problem when they leak fluid. Oil on the rails is a disaster.
Yikes, I can only imagine!
It could be, I suppose, but most of the passenger trains where I am are diesel hydraulic and it doesn't seem to be a problem.
In any case given that the transmission is likely to be in the middle any oil that escapes will probably fall between rather than on the rails.
I'm a mechanic and I didn't even know s***
I was looking throw different type of electric locomotive drive.... and one might suspect, how many different types can there be.... a lot.. REALLY a LOT.
I found like 15 different ways just from a breef look. There is probobly loads more.
And i don´t know if there going to be more parts of this series, but there is also diesel pneumatic.
About electric, as far as i can find there are 5 main categories.
AC-AC
AC-DC (yes, like the metal group)
AC-DC-AC
DC-DC
DC-AC
For something like AC-DC-AC that is the most common today, there is at least two different subtypes, syncronic or asynchronous motor
I don't think I've ever heard DC-AC before. Anti-rectifiers? How does that work?
@@russellgxy2905 typically its a 3 pole H bridge with transistors or thyristors.
If made with transistors they can be feed backwards to be used as a rectifier. This is actually more efficent then diods. So most modern locos have H bridges on both side. Both to the motors as well as the power line. This way they can feed the power back put when breaking.
All diesels are liquid powered ffs.