Measure speed of slot car
I don't see why not, fancy ones only go about 15MPH.   So you just need two IR sensors far enough apart, maybe over the track so you can measure two cars at once.  Then run a timer on the audrino, and record when the first beam breaks, then the second.  Subtract and use difference to calculate the speed.

You could embed one side of the IR detector in the track and put the other above, or maybe put one to the side and the top at an angle.  Just something to let the other car slide by.