The inter-baseboard gap might have been filled but it showed up another problem - the ground level between the two boards included a step. Not a problem for the scenery but those finescale locos are as good as (proper, pre-CGI) Daleks at working the stairs so this needed fixing.
Digging out an old chisel and honing the edge on a stone, I set about shaving the extension top away. I didn't use a best chisel for this as I wasn't entirely sure what I'd find in there. A stray nail wouldn't do the cutting edge any favours.
After half an hour I'd stripped away several layers of plywood and had a feeling the base was below the level of the trackbed bottom. The resulting surface isn't pretty but I'm sure the track won't mind too much. As long as those rail tops are level and smooth, the trains will run.
The short panels of PCB track are just long enough to allow us room for an extra wagon. A couple of sleepers removed from the end of the old track should let the join be tidy. Hopefully all this can be done without too much damage to existing scenery.