Activator rail is a non-full block, meaning that you are passing through it when you "walk" on it. My guess would be that the "on walk on" event scans the block that has Y value 1 less than the player triggering it every time he moves; thus it scans whatever the rail is laid on, instead of the rail. Even though of course human would say you are walking on the rail, the computer only sees you as passing through the block of rail, and _literally_ scans the block below.

This behavior should be able to be avoided if you use plain "on walk" instead, and use a condition "block at player is activator rail", or something similar.
Hope that helped. If not, sorry for wasting your time. Cheers.

 

on step , and on move the same events

 

on step checks the block BELOW the player means you would need to do

Code (Text):

1.  
2. on step:
3.   if block at player is activator rail:
4.     send "ok"
5.