Would it not be easier to create a giant swappable battery pack on a pallet?
If you took a industry standard 1M pallet, and loaded it with approximately 1 cubic meters worth of battery, you could presumably have enough energy to push a train a reasonable distance. Therefore all you would need is to have a replacement battery waiting at the point where you need to replace it. It could be quickly swapped out with a pallet truck, and the train would be ready in minutes rather than hours.
You would lose a chunk of cabin space for this, or would have to make the train a bit longer, but it might be worth it.
On long lines where one a single battery pack is not enough to get it from end to end, you could stop on route and pick up a a replacement fairly easily as well. In fact there would be energy savings in swapping smaller batteries enroute rather than carrying the whole lot around at once.
With enough battery packs you could have a reliable solution for rural lines. Also, you wouldn't necessarily need to lay on extra power to the recharge points, as all the packs could be trickle charged overnight and stockpiled ready for use.
Question is, (and I'm not qualified to answer this) how much energy would such a battery pack contain, and how much does a train need to move on a typical branch line?