Hydraulic Transmission

Motorized rail vehicles with diesel-hydraulic powertrains are powered by onboard diesel engines and propelled by hydraulic transmissions. This transmission type is characterized by the use of one or more torque converters, which are enclosed devices filled up with hydraulic fluid. A pump on one end is driven by the engine, whereas a turbine on the other end is driving the wheels.

Rather than being mechanically linked, the engine and the wheels transfer energy between each other through the fluid. In this setup, the engine is able to run independently of the wheels (it cannot stall, as the fluid is not dense enough to force it to), yet the wheels are still set in motion by it.

With this type of powertrain, the throttle controls the engine speed. With the reverser set in gear, as the engine spins up the pump, it spins the hydraulic fluid inside the torque converter, in turn spinning the turbine on the opposite end. As the turbine is mechanically linked to the wheels, this propels the vehicle.

Hydraulic transmission may include multiple torque converters with varying volumes, with the fluid automatically flowing from one to the other, depending on the vehicle's speed, for best performance.

The heavier the train, the more difficult it is to spin the turbine, and so the more it ends up lagging behind the engine pump. This difference creates friction in the hydraulic fluid and warms it up.

Depending on the session difficulty settings , torque converters can fail catastrophically if they overheat .

Hydraulic transmissions are also characterized by their hydrodynamic braking ability.