The Real Problem Is...
Steam engines don't share the torque between the wheels like Electric Motors do.
If one wheel slips on a Steam Engine, all the torque and all the power and all the load goes to the other wheels, which then break traction and the whole engines is spinning it's wheels. To stop this, steam engines use sand (or even gravel) on the track, and must be heavy in proportion to their power. Which means they must be very heavy, and the track and bed must be very expensive.
If one wheel slips on a (Diesel) Electric, the torque to that wheel is automatically reduced only enough to so that it is not slipping. Load is not flung suddenly onto the other wheels, the engine can be cheaper and lighter, and the road-bed can be cheaper and lighter.
Furthermore, the Diesel Electric engines can be strung together and all share the load correctly. This never worked very well with Steam Engines: it works very well indeed with Diesel Electric.
This is not some magical digital control system, it is just the characteristic of the kind of electric motors that are used in Diesel Electric trains, the torque is proportional to the phase angle.
If fluidized coal becomes very cheap, we might see some Turbine-Electric trains. We won't see the 'return of steam engines' because they aren't very suitable for pulling trains.