So I lowered the roof of the water tank a bit. The black stripes are now rails as on the original and the manhole covers are sticking out. The side door has been removed, the open part underneath has been closed off.

The tanker we initially received was for a different station, the one we got actually has shelves behind the doors. (A 3x6 door does exist but I'm not shelling €80 for it on Bricklink. The doors in the picture are like €0,36 or so.)