The third wheel hidden in a bike!

Clue No.1 on How a bike works

We call it a bicycle because it has two wheels.

But in reality, a bicycle has 3 wheels.

This wheel runs on a track, same like a train.

A train can travel anywhere on a train track. But in reality we can't lay tracks everywhere. To solve this problem, we can incorporate the track inside a wheel. That is what we have in a bicycle.

A = track (infinite),  B=Wheel

This is only a clue, not the solution.