I expected that a TNT cart would be primed when pushed off of an Activator Rail.
When pushed or riding off of an Activator Rail, a TNT Cart would immediately explode, regardless of if it had redstone power or not. Also, if kept on an Activator Rail when primed, it will explode as normal TNT would.
1.Place an Activator Rail with a TNT Cart on it with or without redstone power
2.Push the cart off the Rail