Some Of The Weirdest Trains Ever Used Car-Like Torque Converters To Gain Efficiency

Most of the freight locomotives here in America use diesel-electric technology. They have gigantic diesel engines that act as generators for the electric motors that do the heavy lifting to get power to the rails. Several decades ago, American railroads thought they could make trains more efficient and cheaper to run by ditching the electric motors for torque converters and a direct-drive transmission. These were some of the weirdest trains to ever prowl America, but they didn’t really work well here.

America’s railroads have spent over a century with a near-unquenchable thirst for power and efficiency. America has a beautifully unique geography, featuring everything from long and flat plains to some of the most grueling mountain passes, and railroads have long wanted to conquer them all. Out west, the Union Pacific beat the Wasatch Range down with horsepower. Meanwhile, Southern Pacific concerned itself with Donner Pass and Tehachapi Pass while Denver & Rio Grande Western also had to battle its own mountain routes.

For decades, railroads found various novel ways to conquer their roads using some of the most brilliant minds in railroading. The most famous example of this came from the efforts of the Union Pacific. The railroad’s quest for domination led to the mighty Big Boy steamers, the absurd bridge-melting Gas Turbine-Electric Locomotive, and later, the gargantuan EMD DDA40X, but that’s not all. Union Pacific even dabbled in turbine-electric locomotives.

While much of locomotive history has focused on the downright bonkers experiments performed by Union Pacific, it wasn’t the only railroad trying to engineer its way out of problems. Back in the late 1950s, Denver & Rio Grande Western and Southern Pacific (SP was UP’s rival) were looking for the next best thing in diesel power. At the time, Trains.com writes, these railroads were using 2,400 HP Alco RSD-15s, EMD SD24s, and FM H-24-66s. While strong, it was found these locomotives weren’t ideal for the longest and heaviest trains. Other locomotives on the roster included EMD F7s and GP9s, which were known for being lashed up in over a half-dozen units to haul heavy freight through grades. Doing this was expensive in both fuel and maintenance costs.

Both of these railroads thought that there could be something better. What if they could find a single locomotive that was so powerful that only a few units could do the work of many of the existing locomotives? At the time, Union Pacific was experimenting with forced induction and gas turbines. For Rio Grande and Southern Pacific, the potential solution came from Germany.

The Quest For Power

Back in the 1930s, the Deutsche Reichsbahn (also known as the German National Railway) faced an interesting problem. It was working on advancements in diesel technology, but it hit a few roadblocks. In those days, some European diesel locomotives weighed well over 100 metric tons, which just wasn’t going to work for large, high-speed applications. Many diesels also didn’t rev particularly high. Germany had seen success in smaller diesel locomotive applications, but these didn’t quite scale up as desired.

The solution was something that hadn’t been seen at this scale before. In 1935, locomotive builder Krauss-Maffei took a MAN supercharged engine, coupled it to a torque converter, and had that power sent to the wheels through a Voith transmission. The results were tremendous. This locomotive punched out an impressive for its day 1,400 HP, achieved a top speed of 62 mph, and did so with a weight of only 75 metric tons, or a massive weight savings compared to other large diesels of the day.

How this locomotive worked was fascinating. In a typical diesel-electric locomotive, the diesel engine isn’t actually driving the axles. Instead, the beast of an engine is a prime mover that is more or less converting diesel fuel and mechanical energy into electricity. Electric motors attached to the axles and wheelsets directly drive the axles forward and backward. Typical diesel-electric motor setups involve a gear on the motor turning a gear on the axle, and that’s how you get your reduction.

Sometimes, railroaders may call the traction motors the “transmission” of a diesel-electric locomotive, but in reality, the complete package is closer to the basic premise of an extended-range electric car.

How Diesel-Hydraulic Locomotives Work

Diesel-hydraulic locomotives work on a different principle. The diesel engine is still there, but the electric motors are not present. Instead, attached to the engine is a large torque converter. This isn’t the exact same thing that you have in a traditional automatic car, but it does work in a similar way. The Railway Technical Website, a great resource for learning how trains work, gives this explanation:

Hydraulic transmission works on the same principal as the fluid coupling but it allows a wider range of “slip” between the engine and wheels. It is known as a “torque converter”. When the train speed has increased sufficiently to match the engine speed, the fluid is drained out of the torque converter so that the engine is virtually coupled directly to the locomotive wheels. It is virtually direct because the coupling is usually a fluid coupling, to give some “slip”. Higher speed locomotives use two or three torque converters in a sequence similar to gear changing in a mechanical transmission and some have used a combination of torque converters and gears.

Some designs of diesel-hydraulic locomotives had two diesel engines and two transmission systems, one for each bogie. The design was poplar in Germany (the V200 series of locomotives, for example) in the 1950s and was imported into parts of the UK in the 1960s. However, it did not work well in heavy or express locomotive designs and has largely been replaced by diesel-electric transmission.

The video above explains some more.

As the Railway Technical Website notes, diesel-electric and diesel-hydraulic aren’t the only ways to propel a locomotive forward. Many smaller locomotives, especially some found in developing regions of the world, use a fluid coupling on the back of the diesel engine and then a mechanical manual transmission. There’s no clutch pedal thanks to the fluid coupling, but diesel-mechanical designs can operate like a car with a manual transmission, where the engineer has to shift gears, choosing the best ratios for the job.

There’s also the Diesel Multiple Unit. These types of trains involve railcars where some or all cars will have their own diesel engines, often mounted under the railcar’s floor and utilizing a mechanical transmission. The most common DMUs in America, and the most common type of diesel-hydraulic trains to be produced in America, were the Budd Rail Diesel Cars (above) of the late 1940s to early 1960s.

There are a few huge theoretical advantages to diesel-hydraulic locomotives. One benefit is that these locomotives weigh dozens of tons less than a diesel-electric, which can be beneficial, such as in the case of higher-speed passenger service. Another benefit is that due to the torque converter, a diesel-hydraulic locomotive can produce full tractive effort right when starting off. Diesel-hydraulics also accelerate smoothly, have less wheelslip under load, and, important to railroads in America, they don’t experience the overheating that traction motors do when they’re overworked.

As Trains.com writes, other perceived benefits came from the potential for high reliability, simplified controls, and potentially easier maintenance. Another benefit of the diesel-hydraulic locomotive was that it could cross somewhat deep water without the worry of shorting out electrical equipment. In other words, Germany’s diesel-hydraulic trains were pretty much hot rods in their day. They were fast, powerful, and didn’t weigh as much as early heavy diesel-electrics.

As such, the technology made its way to the United Kingdom, where the twin-engine Maybach-powered British Rail Class 52 “Westerns” of the 1960s would become icons in the region. Diesel-hydraulics also made their way to Spain.

One Diesel To Rule Them All

Here in America, Denver & Rio Grande Western and Southern Pacific were dealing with diesel-electric equipment that was aging out, and the realization that none of the commercially available locomotives in America were powerful enough for their tastes. So, they turned to diesel-hydraulic pioneer Krauss-Maffei for locomotives to dominate the mountain ranges.

Krauss-Maffei came out of the other end with the ML-4000. To call these locomotives beasts would be an understatement. They used a pair of high-revving 86-liter Maybach MD870 V16 engines, making 1,770 HP each. These were bolted to torque converters and Voith transmissions. On the rails, they were good for 78,000 pounds of starting tractive effort and 106,000 pounds of continuous tractive effort. These 344,000-pound weirdos cost the railroads about $492,403 ($5,300,685 in June 2025) when they hit the rails beginning in 1961.

Testing was initially carried out in Europe, where Denver & Rio Grande Western and Southern Pacific were impressed. The locomotives were then loaded onto a ship, sailed across the Atlantic, and offloaded in Texas in October 1961.

At the time, these diesel-hydraulic locomotives were more powerful than any commercially available diesel-electric locomotive in America, and in theory, just a few of these monsters could haul the kind of train up a mountain that might have taken up to 10 older diesel-electrics.

At first, the two railroads took on just three units each, which satisfied Krauss-Maffei’s demands. Southern Pacific put its ML-4000s to work on the Sierra Nevada mountain range. D&RGW put its trio to work on the mountains as well. However, as Trains.com writes, the D&RGW discovered that the weirdo locos weren’t any better at the job than its new EMD GP30s, and in 1964, D&RGW ended its diesel-hydraulic program, selling the locomotives to SP, which still tried to make the tech work.

For those counting, D&RGW didn’t even give the locos five years before giving up on them.

The Diesel-Hydraulics Show Cracks

SP, now with six ML-4000s, put its diesel-hydraulic units to work hauling freight through Donner Pass and Tehachapi Pass. Yet, SP ended up arriving at the same conclusion as D&RGW. While these new locomotives might have performed well in Europe, they just weren’t coping well with the heavy demands of American Class 1 railroad work. The six prototype ML-4000s were scrapped in 1967, but SP wasn’t ready to give up yet.

For the next iteration of the ML-4000, SP wanted the locomotive to become more American in design. This called for a road-switcher body (a locomotive that’s good for both shunting cars in a yard and for pulling freight on the road), the capability to lash multiple units together, and an American medium-speed diesel prime mover. Reportedly, SP also demanded that the new ML-4000s be designed so that they wouldn’t require a special servicing facility like the prototypes did.

Krauss-Maffei delivered 15 of these new-style units in 1963, but sort of missed the mark. The locomotives didn’t come with American power and were still fitted with a pair of German Maybach MD870 prime movers. But at least now they were tuned to 2,000 HP each for a total output of 4,000 HP. Though, reportedly, in service, the locomotives were set to 3,000 HP to 3,450 HP. These engines were then bolted to Voith direct-drive hydraulic transmissions. Krauss-Maffei did understand the assignment somewhat by giving the ML-4000s a more American-style body and standard General Steel Casting trucks, which were found on Alco units back then.

Unfortunately, the changes weren’t enough to save the ML-4000 series. Not only did the ML-4000s fail to meet the mark on performance, but SP experienced reliability issues, including with the Maybach engines and with driveshaft failures. As a result, 14 of the 15 new ML-4000s were scrapped in 1968. Of the 21 ML-4000s that were ever built, only one has survived to be preserved. That one is in the hands of the Niles Canyon Railway in California. Check out the video embedded above.

Third Time’s The Charm?

Amazingly, SP didn’t put all of its eggs into the Krauss-Maffei basket. The railroad also commissioned three locomotives from the American Locomotive Company. The Alco DH643, three of which were delivered to SP in 1964, corrected some of the faults of the German locomotives. Power came from a pair of Alco 12-251C 131-liter V12 engines producing 2,150 HP each.

These engines were mounted facing opposite each other, placing their radiators in the middle of the locomotive body. This was great for SP, which now finally got the American-powered diesel-hydraulic locomotives that it had been begging for. The Voith hydraulic transmissions made a return here, now produced under a license from Germany. In essence, these locomotives were basically two locomotives grafted together to create a single mammoth unit. Power was sent down to heavy-duty trucks, and reportedly, an Alco DH643 could haul twice the freight of a diesel-electric locomotive with the same horsepower rating.

Reportedly, the DH643s did perform well in long and heavy freight. So what did these ones in? Well, there were still a few outstanding issues. SP still had to deal with the fact that several components were sourced from overseas suppliers, which added one layer of complications to repairs. Reliability issues then stemmed from the fact that brass shavings were found in gearboxes, necessitating costly repairs in the three units. SP didn’t order any additional DH643s and retired the only three in 1972.

Products Of Their Time

Ultimately, what really killed all of the American diesel-hydraulics was the march of time. By the mid-1960s, American locomotive builders had caught up with simpler technology. The EMD SD40 diesel-electric locomotive, which launched in 1966, made 3,000 HP with only one V16 prime mover. The EMD SD45, which was built during the same era, managed 3,600 HP with a single V20 prime mover. Add in the General Electric U30C and U33C, and now there was really little reason for SP to continue down the diesel-hydraulic path.

Despite that, diesel-hydraulic technology never really died. Diesel-hydraulic locomotives are even produced today, with examples coming from John Cockerill of Belgium, San-Engineering of Bangalore, and Atlas Copco GIA of Sweden. These locomotives aren’t thundering up mountain ranges in America, but they do put in work in railway maintenance and industry.

What’s wild about the American diesel-hydraulic locomotives is just how quickly they faltered. The first six survived only six years on the rails. The second set of 15 got to live for four years. At least the Alco models got a whole eight years of existence, but it’s sort of shocking that none of these locomotives even got to perform for a decade.

Yet, I can understand why they existed. At the time, diesel-hydraulic locomotives were the best way for a railroad not named Union Pacific to crank up the power. But it didn’t take long for America’s locomotive builders to catch up. So, the diesel-hydraulics were probably destined to be a short-lived bridge between weaker first-generation diesel-electrics and the powerhouses of today. But for a time, some of the weirdest trains in America had torque converters.