Thanks for the extra info F de Graaf!

I guess the more area you have touching the rails the more friction there is, and the harder it will be for the wheels to slip. But - is there not also a counter-acting effect? The more wheels you have down, the more the weight is spread out, so the less downward force you get on each wheel - hence each wheel will slip more easily? I guess the question is which effect is the most significant?

Bart.