A Gravity train (https://en.wikipedia.org/wiki/Gravity_train) goes through a tunnel inside a planet that connects point A with point B. On Earth, the train would not gain enough impulse to reach the end point due to the loss of energy mainly by friction.
However, I have read some people suggest that this could be feasible in smaller planets like the Mars. Is this true? Could the train reach the end point of the route in a smaller planet (even if it has an atmosphere) somehow?
Sure. Put a rocket on it to pay for any energy losses to friction, tidal forces, etc, and do it on a planet with a core that won't cook you (a nice round asteroid like Ceres would do nicely). A "gravity train" is just an elliptical orbit with an eccentricity of exactly 1. The problem with such an orbit is that there's usually a planet in the way. That's probably an insurmountable engineering challenge for anything large enough that you couldn't easily jump and swing from one side to the other with a teather, like a microgravity Spiderman, but no problems from first principles.
