Has anybody experienced a problem with the code detailed for the first 'play' button? I keep ending up with the following error report: 1120:Access of undefined property play_btn.............
The result being that the stop command does not apply with the cars continuing their motion on a loop; with no response from the 'Play' button??
I don't know what example (or whatever) you might be referring to.
For the error you are getting, make sure you have assigned an instance name "play_btn" to the button. Triple check it. You assign the instance name of an object in the Properties panel when the object is selected.