Power Logistics

Carry electricity by train. Charge an accumulator wagon at a station where power is plentiful, drive it to an outpost, and unload it there. A wagon can even restart an outpost whose power is completely dead.

The loading is done by arc towers, which need only a connection to the electric network.

How it works

Accumulator wagon. A flatbed carrying banks of accumulator cells. Its recipe holds the accumulators, so they're already built in. It holds 500 MJ by default. Five bars on it show how charged it is, and clicking it shows the exact amount and what the towers are doing.
Arc tower, charging. Stand it beside the track and connect it to your power poles. A stopped wagon beside it fills from your network, with an arc jumping between them. It draws power like any machine, so accumulators can feed it.
Arc tower, unloading. Press R on a tower to swap it to unloading. It empties a stopped wagon into the network, like a generator. That works on a network of only accumulators, and on one with no power at all, so a full wagon can bring a dead outpost back to life.
Which way it faces. A tower turns itself toward the nearest rail, on whichever side of the track it stands. Charging towers glow blue and unloading towers glow yellow from every side.

Rules that keep it simple

A tower works on the nearest stopped wagon whose middle is within 2 tiles of the tower, so it never reaches the next car.
One train is never charged and unloaded at the same time. The first tower to act claims the train, and a tower of the other mode waits (alt-mode shows why).
Towers within 13 tiles of each other always share a mode: rotating one swaps the chain, and a new tower takes the mode of one nearby. A charging and an unloading tower can't fight over a wagon.
Charging loses a little (80% by default). Unloading loses nothing.
A wagon's charge is lost if it is mined or destroyed, like an accumulator's. Don't carry charged wagons around in your inventory.

Train schedules

The wagon holds "Wagon charge" items, one for each 1% of its charge, so the train's own wait conditions work: Cargo full (fully charged), Cargo empty (under 1%), or Item count of Wagon charge for anything in between. The items can't be taken out or put in.

Good to know

Research "Power logistics" (red, green and blue science, after advanced circuits).
Settings (startup): wagon capacity, tower power, charging efficiency, and whether a charging tower draws like a machine (default, so accumulators can feed it but it shares a shortage with your machines) or takes only surplus power (never starves anything).
Two wagons fit in a rocket.
Works with or without Space Age.