I agree. In-motion charging might be introduced later, perhaps with a package of targeted supply-strengthening measures on the long term DC parts of the network. That might be a way to improve utilisation of the fleet in future as passenger numbers increase, if shorter terminal layovers could be tolerated.
In the meantime, it's a great benefit to be able to store braking energy. While I'm sure older DART units can inject regen braking current back into the supply line, that only works if the local line is receptive, with other train loads or grid-exporting substations able, at that precise moment, to use the energy. If the line is not receptive, trains must burn off the braking energy in resistor banks, if equipped, or incur friction brake wear instead. Batteries provide another local destination for that energy, where it can be used later.
A battery has its own receptivity constraint, its charge capacity. In planning for battery operations, a train leaving a branch terminus at high altitude for example, facing a ten-mile descent to the coast, would ideally not leave with a full charge. A custom charging plan for the duty could take into account the likely energy recoverable from normal regen braking on that major gradient, with the twin aims of maximising energy recovery and minimising friction brake wear.
