Kicked out/Can't get in minecart when it is on a powered activator rail.

I built a railway and tested it out with a minecart with chest and it worked just fine. The activator rails pushed it along as they should, but then when I got a normal minecart and put it on the activator rail I couldn't get in it and it was shaking as if someone were trying to break it. I de-powered the rail and it stopped, but then when I powered it again I got kicked out of the minecart and couldn't get back in.