How can I make my wagon go uphill on a roller coaster?

Hello, I am facing a problem

I have tried a lot and it seems to be simple, but I can't get the problem.
(2D Game)
I have a wagon that I want to follow the path of the roller coaster.
I have set collisions for everything, I have a rigbody for the wagon, I added gravity and now want to apple force.
But going uphill makes the wagon go very slow and stop there,
I want it to go uphill with a constant speed of 2 and make it stop at the very top.

I can't get the last part, it always stops when trying to go uphill.