Flat belts can be surprisingly clever at staying on the right place, but perverse if you don't keep them happy.
Would you expect adding flanges to make them more or less happy?
Each shape of wheel comes in a matched pair. You can pick a matching pair that you think most likely to work nicely and fit the belt over them.
Now press the yellow plunger to run the belt and watch whether it stays obediently centred or mischievously climbs towards the edge.
Things become more interesting when alignment happens to be less than perfect. Hence the wheels at the back are spring-loaded from below, so you can push any one down with one finger.
Shoehorned into the large yellow box with flanged plate lid is a battery box, plunger switch and motor with drive to four vertical axles, all in genuine unmutilated parts, including the switch. It took some trial and error before I found a way to squeeze that lot in.
My appreciated goes to Meccano Limited for sourcing such lovely flat belts (!), and thanks to Mike Gleen for letting me snaffle a few for 'scientific purposes'.