[Big Dave]

Technically, its a snap. The basic technology has been around for more than a century. Its proven, VERY durable, and readily available.

At the yard diesels would make up the train under the end of the catenary. If necessary, hostlers would used diesels to get the electric consist onto the train and under the catenary. Some electrics had "hostler" diesels for manuvering themselves around in yards. They may do that again.

In relatively flat terrain, the electrics will draw catenary power until the train tops the "ruling grade" and then will try to coast the rest of the way. Pulse-and-glide" on an enormous (10,000 ton) scale. Railroad engineers have been doing "pulse-and-glide" for a century and a half.

If the have to slow down, the first step is regenerative braking. The engineer rotates the motor fields ninety degrees and the motors become alternators. The power generated goes back into the catenary if another train within a few miles needs the power. Diesel electrics use regenerative braking, but as they have no place to send the regenerated power, they simply waste it over resistors.

On big grades (all railroads have at least one one their mainlines) a dispatcher will hold trains at the top until one starts up. The downbound train makes a bunch of regenerative power and mostly powers the upbound train up the grade. That way the second train uses only 15% of the grid power the first one did. Very energy-efficient.

Modern electrics would be even more efficient as we now have variable frequency drives (VFDs) of sufficient capability for 10,000+ HP locomotives. Old electrics had to convert to DC and use resistance speed control. New electrics would receive 60Hz power (rather than the 25 Hz used earlier) and the VFDs would simply send power to the motors at a reduced frequency for the desired speed.

Because railroad movements are centrally controlled, the main dispatch center could also order power generation up and down as necessary. To a limited extent the railroad could dovetail their movements with the grid requirements to even out generation. For instance trains could be somewhat limited at peak power usage hours (12 noon to 7 PM local) but go like mad at night and in the morning.

But to dovetail operations, electric power serving the railroads absolutely must be dispatchable on demand. You can't let your trains stop because the sun went down or the wind died down. A couple instances of that and the diesel comes back big time.