Luckily, during construction, I took into account the need to facilitate easy battery replacement. The battery pack fits into a recess cut into the ply floor of the railcar, and is held in place by a brass strap, which is retained by two wood screws.
The first job when replacing the battery was to isolate the power - to avoid a short circuit when metal tools came into contact with any live parts - this was achieved by removing the fuse. Once the wiring was removed from the retaining clips on the brass strap, it could be removed. The battery pack was then levered out of position and the positive wire was disconnected from the fuse holder; the negative was removed from one of the terminals on the charge socket.
The new battery was then popped in and the positive and negative wires soldered on to the aforementioned terminals. That done; the brass strap was replaced and the loose wiring tidied up again by securing with the retaining clips.
With its new battery, the railcar should deliver about four hours running on a full charge.