[Graham]

That's a really great illustration Andy, many thanks.

But I also think "thickness" of the guide flag is equally as important, especially when the joints of track pieces pinch together because of the connections. 

Club based "wood" guides are just not going to work on these plastic tracks, and if the host has to try to reduce the thickness they are faced with having to remove the guide and grind away the sides (which in itself can weaken the guide).

The Classic SCX,  Scalextric or Scalextric Sport track has a maximum slot width of ~2.25mm, so fitting a 2mm thick guide is asking for problems.