If, however, you are driving the vehicle, it works as it should. Only if there’s no driver it doesn’t work.
Any help would be appreciated. I’ve tried setting the network owner to the player but still no luck.
The cart has likely “fallen asleep” and is no longer being simulated by the physics engine. This is not the case when you are actively riding it, which is why it works as intended. There are a few ways to “wake up” sleeping assemblies, but the easiest I’ve found is to toggle the Anchored property of its root part on and then off again.
I think you may want to consider setting the network owner to the server in order to restimulate the physics engine.
Alternatively, you could just tween the cart itself too, though this would require a few welds.
I should have been more specific; the cart needs to be anchored and then unanchored by the tweening code right before it begins to tween the part size.