Accumulator wagons for transporting power between different power grids.
When they stop, the wagons will activate themselves as accumulators, helping to move power around between disconnected power grids.
Normally, they have the same priority as normal accumulators, only consuming excess power and distributing power to machines at the same priority level. Two custom circuit signals are available to instruct the accumulator wagons to change their priority when they stop at stations - if charging is prioritized they'll consume power from other accumulators, and if discharging is prioritized they'll send all of their power into the network.
When stopping at a station, the accumulator wagons will check for circuit signals on the station that they're arriving at - if a signal is present at the station, wagons will have their priority set appropriately.
The wagons contain a special fluid representing battery charge - this fluid can be removed from the wagons for storage, and pumped back into them to recharge.
Thanks to Arcani for creating the idea, and graphics assets! Translations and other contributions are welcome, preferably through GitHub.