It's interesting that European railroads have developed locomotives with hydrodynamic transmissions to the extent that they have, yet such locomotives are practically non-existent in North America—I've never seen one. On the face of it, it would seem hydrodynamic power transmission would be better than electric—no traction motors to overheat and burn out. Also, the control system can be much less complex. Not to be overlooked is the technical skill required to maintain and repair what is basically a scaled-up version of an automobile's automatic transmission is much less than required with a Diesel-electric unit.
However, the operating conditions to which freight (goods) locomotives on Canadian and American railroads are subjected are much more severe than seen in most other parts of the world. Train lengths exceeding 125 cars are common, and train weights often reach 15,000 tons when hauling bulk commodities. It takes an enormous amount of power to haul such a train, especially in the moutain districts.
As an example, an EMD SD70ACe freight locomotive develops 4500 horsepower and 157,000 pounds of continuous tractive effort. I've seen trains with as many as six of these locomotives in use, usually four pulling and two pushing. The forces involved are mind-boggling. While a hydrodynamic transmission could be made that would withstand that level of power, the transmission's output has to be transferred to the trucks (bogeys) and ultimately to the wheelsets. To date, no satisfactory method of doing so has been developed for high-horsepower freight locomotives.
