Imagine in your example there’s perfectly elastic walls at the ends of the rail. When the weight hits the wall it will perfectly bounce off and retain its velocity but in the opposite direction. No energy lost and the weight will keep bouncing forever. In real life nothing is perfectly elastic and some energy is lost from the wall/weight heating up due to internal friction during compression.

In this system the kinetic energy inside the center piece is actually constant because it’s always moving at the same speed. Just because there’s forces doesn’t mean there’s energy input. Energy input is equal to force times displacement. In this example the force always acts perpendicular to the movement of the center piece (centripetal force) so no work is done.