Messing around with rails, placed a minecart on powered activator rails and tried to enter it. When I entered, I was in the floor. After trying to recreate it, I found that it didn't happen every time. Basically, the steps are:

1. Place down activator rails.
2. Activate the rails with a lever.
3. Place a minecart on the rails.
4. Enter the minecart.
5. If you are not clipped into the floor, repeat step 4.
6. If step 5 yields no results, exit and re-enter the world and repeat step 5.