To put the 1000 words that 5 Kids' pic represents, the different holes are for using different timers. I presume that Best Track drills the three sizes of holes by default.
And, not really intending to highjack this, but how are the sensors supposed to be mounted? Our timer (sorry, I cannot remember the brand off the top of my head) uses light sensors under the track picking up overhead light; when a car passes over, the signal is cut, and the time is marked from that. The bridge with the place and time display is a separate piece, connected by a cable.
Our sensors are on a green breadboard strip. I've been setting up the track and putting the sensor strip on top of a stack of cardboard to hold it against the bottom of the track. It is still very prone to being moved and the timer stops working on one or more lanes because the sensor has moved into shadow.
Another issue, at least in my mind, is that the sensors are recessed under the track by the track thickness (minus 1/4" height of the sensors).
Breadboard is similar to this (the closest pic I found online):
Is there a way of mounting this such that it is
mounted and doesn't move?
TIA,
-Eric