With the third rail dead by Carl's own hand, the brick-sized dwarven cells are the only thing still turning the carts' wheels.
Carl built a stockpile of these on purpose. Brick-sized dwarven cells, four mana potions to a charge, produced in quantity by the Battery Fabricator he bought with westerns and moonshine — and the reason the whole rescue could run on hardware whose power supply nobody controls.
It paid off immediately. Carl's train bomb sealed the Vermillion Line tunnel and, in the process, appeared to knock out the line's power. That left the party's remaining Interdiction Rapid-Response Railway Repair Cart with no third rail to draw from, which would have been a serious problem for anyone else. "We had to run the carts off of battery power, but that was fine. I now had plenty of them."
Everything downstream of that sentence depends on the batteries: the cart corked into the tunnel mouth whose Debris Scoop swallows every Blister Ghoul that walks up the track, the second cart facing the other way with two fresh hands lashed to it, and the party's ability to run the portal on and off for hours of controlled grinding. A cheap dwarven power cell is what turns a dead rail line into a killing floor of Carl's choosing.
Plain brick-sized dwarven power cells, charged four mana potions at a time, stacked by the dozen in Carl's inventory.
The independent power supply behind every cart the party still operates, once the Vermillion line's own power died.