Yeah, I knew the cost was the main reason, but I figured they couldn't put enough power though the line in order to handle such large trains going like 70 MPH. :?
There is so much more power in electric lines that you couldn't produce with any realistic number of Diesel generators (except a really really large one like on a ship).
The problem is just, that there have to be transformer stations "everywhere" to keep up the voltage. So it doesn't make any sense on tracks that aren't very busy or where there aren't any good connections to the electric grid.
I know, that here in some "rather" [nothing compared to the US] rural areas with electric tracks, there are gas-turbine power generators that produce electricity.
And diesel trains are diesel electric, because an electric motor is just better if you need a lot of torque at low rpm.
What I personally find really interesting and what must be really complicated to develop, is that "modern" electric trains like a ICE 3 use recuperation. Means if the train brakes, the motors, that are mounted in the wheels, are switched from motor to generator mode and they generate electricity. That electricity has to be transformed to the voltage of the train power line AND synced with its modulation.
And this while braking, meaning that rpm of the wheels change constantly which means that the generators output changes also constantly.
I don't know too much about electricity, but it must be really complicated to make that work flawlessly.