All diesel locomotives are diesel-electrics, possibly with the exception of some smaller DMUs. A diesel engine is about torque. For example, Volvo's D16E660 makes approximately 640 bhp, but 2286 ft-lb of torque. Another example is the VW Passat 2.0 litre diesel from a few years back, which made 136 hp, but 248lb-ft of torque. For an equivlency, consider their base gas motor from the same car, a 1.8 litre turbocharged unit making 170bhp, and 166lb-ft of torque.
Amtrak's P42DC, for instance, makes 4,200 Bhp. I haven't found the torque figure, but it relative to the Volvo, it makes 15,000 lb-ft of torque, approximately. That kind of torque would bust the hell out of any kind of multi-geared transmission I can think of. Electric motors, on the other hand, because they are comfortable with spinning at pretty much any speed from 1 rpm to 13,000 RPM don't really need a transmission unless you want a combination of high top speed and quick acceleration, such as with the Tesla electric car.
So instead of transmitting the power to the wheels via a drive shaft, they use a generator to turn it into electricity. Then they transmit the electricity from the diesel engine to electric traction motors on each bogie or axle (not sure which), which then turn the electricity back into torque and motion. The electrical components merely serve as a transmission, if you will. There are some newer experiments using batteries to recapture electricity, but they wouldn't make much difference. Especially on a screamer engine like Amtrak's with HEP that requires the engine to turn at a constant rpm, and thus you don't save much due to lower loads on it.