Activating Minecart with Command Block with an Activator Rail causes multiple activations

If you use the Minecart with Command Block and an Activator Rail to activate it, the command written into the command block gets activated several times. I hope the screenshots will explain it a bit better than my fairly bad english.

Just a little explanation oof the screenshots:
First one shows the setup, the other two shows the output after the Minecart went over it one time.