Thanks to everyone for their help and comments. In the end the solution was close to home as my son (an N-guager) realised he had one in his collection that may do and it turned out to be a near perfect fit. Some interesting comments about the Airfix auto-tank here; my own experience is that in its early days (when they were new) I replaced the grooved traction tyre wheels for regular smooth ones and had no trouble at all running 2-coach auto-trains up my gradients, which it is doing once again after all these years. It still has its original 6 plunger contacts (but I did get hold of half a do
I am getting my Airfix Auto-tank back to running order but have managed to lose the spring that provides the connection between the bottom motor terminal and wheel contacts; it's on the pin protruding from the bottom of the motor. This must have happened to someone before, does anyone know of a suitable replacement? I was thinking amongst the pony-truck springs, carbon brush springs etc there must be one similar enough.