A walled-in chamber at the end of Bridge Street

At the far west end of Bridge Street there's a little walled-in chamber with something in it, and no way in from the street.
Nudge 1Where to look
It isn't reached from Keldor at all. Something comes in from the left, from underground, very fast.
Nudge 2A stronger push
The mines' old railway has a branch that ends here.
Show the solutionThe whole answer, with a video
Ride the minecart and take its last exit (The minecart): it throws Dizzy into this chamber, where the gravediggers muddy spade lies. Pick it up on the way through: Dizzy can't come back in.