[300SLR]

The magnetic attraction between the magnet and the rails of plastic track depends how on the gap between them.
(To be technical, it follows an inverse square law - although this is something of a simplification when things are very close)
Try putting the magnet on the rails and you'll feel the attraction at its strongest, as soon as there is a slight gap the attraction reduces and gets smaller the further the magnet is moved away.
This can be used in car tuning by moving the magnet up and down to adjust the amount of attraction.
Obviously there is a practical minimum ground clearance under the car, so the maximum attraction is a good deal less than when stuck to the rails.

 

[300SLR]

Magnetic attraction is a force, so it should be measured in units of force.
For slot car use it is usually expressed as grams. (Physicists among us will know that's not correct, it would be better given grams force, but most people just call it grams.)

There is a tool called a Magnet Marshall which is used to measure this force.
Magnet Marshalls are often used in scrutineering for races where the rules limit magnetic attraction.