Creating a cart game, whats the best 'conveyor' method?

I have tracks that 'carts' are going to run on, I don't want the track themselves to be conveyors because people upgrade their carts to be faster, so I want the speed and pushing to come from the cart itsself. I have a script in the bottom part of the cart,

while true do
script.Parent.Velocity = script.Parent.CFrame.lookVector * script.Speed.Value
wait(0.1)
end

Inside of the script I have a NumberValue named Speed.

When the cart flies off the track the cart starts doing donuts in the air and isn't really affected by gravity. I want something to push the cart but keep all of its gravity if that makes any sense. I'm not sure what I could use to achieve this, any help would be appreciated.