How Does a Choo-Choo Mechanism release?

The motors actually keep rotating in one direction. It’s the knob and folding action that passively release the catapult.

Consider this prototype here:

There is a two-part linkage linking the gear and the catapult arm and a “knob” that “folds” the linkage. For this to work, there must be an elastic force acting on the other side of the arm (in the form of rubber bands). Normally, because of the elastic force, the long arm of the catapult naturally wants to go up in a direction parallel to the long arm of the linkage, but is “leashed” by the linkage. What happens is that the knob “folds” the linkage over like this:

The short part of the linkage is pulled by the catapult arm (which again wants to go up because of the elastic force) via the long part and thus wants to go up too, but the knob rotating in the other direction “catches” that and keeps folding it and from going up. The knob folding the linkage pulls down the arm, and thus builds up the energy in the rubber bands.

The knob keeps doing so until the system gets to this point:

At this point, the knob is no longer acting against the direction the linkage and the catapult arm want to go up. The knob thus is no longer restricting that upward motion and the arm is free to go up, actuated by the rubber bands. That’s the point of release.

After the release, the gear keeps rotating and the knob comes back around to catch the short part of the linkage, and the process repeats.

You might also have a look at this video, starting around time = 2:50

That is an excellent answer! Thank you so much! Just one more question. What sort of parts do you recommend for the linkages? I see in your example 1x25s were used but they don’t seem very sturdy, or maybe they are perfect I don’t know.

1x25s are fine because most of the force on them is tension. You may want to support the joints from both sides however.

One of the 1x25s only has tension but the other one also has the force from the catch acting on it. I think I’ll use a c channel for the shorter one and the other can safely be a 1x25.

The catch’s force is not acting in the short piece’s bending direction. It’d be safe to use a 1x25 with the catch.