When placing a redstone block next to an activator rail, the cart will shake and not be able to move or be mounted.
*This was tested in Realms and Singleplayer at the same location. It works fine in singleplayer, but not in Realms.
In previous update, the activator rail, redstone block, and minecart were functioning correctly.
That's how activator rails are supposed to work. You most likely meant to use powered rails instead.