Question

What makes a cart go down a slanted track when gravity only acts downwards

Answer 1

gravity

Answer 2

I dont think you understood the question

Answer 3

Gravity accelerates the cart. The ramp acts as a machine, changing the direction.

There are two forces affecting the cart. Gravity is directed downward. There is also the normal force exerted by the ramp on the cart. This will counteract some of the gravity upward as well as "push" the cart sideways, resulting in a diagonal acceleration down the ramp.