My ideas as of right now, some recycled from the original mod page, some new:
An artillery wagon that carries 20% more shells than a regular one, and fits the electric theme in general.
Passenger wagons that look like bullet train cars and can carry a couple of players (4, maybe configurable).
A battery wagon that doesn't provide any motive power but can be charged if parked right next to a battery charger and powers the engines when charged, providing an expensive alternative to battery pack logistics.
A "regenerative braking" research that allows electric trains to force power back into any attached battery wagons when they apply their brakes. Has multiple levels, starting at perhaps 15% power recovery and capping at 50%.
Some kind of "alkaline battery pack" that acts as a fourth option. They're a lot cheaper than the other batteries, and are extremely energy dense with modest acceleration and unchanged top speed, but CANNOT BE RECHARGED, only refurbished.