The payload jumps the rails and runs through the dirt where it goes half as fast.
-or-
The payload track passes right next to, on top of, or crosses a live railroad track, and when the payload itself gets close enough, trains ala traingrid.
-better yet-
The cart is on the ceiling.
-alternatively-
When 5 or more people are pushing the cart, it creates a bullet blocker brush surrounding it.
-also-
The cart has another cart following it with a teleporter hardcoded to it.
-not to mention-
If the red team inflicts too much explosion damage to the cart, they lose.
-don't forget-
The cart starts next to the red base, and the blue team needs to steal it.