The trap door for GraviTrax is a 1-way switch, where the first marble drops through a hole in the tile, and all subsequent marbles go straight through the tile. The door is resettable from above using little "handle" that protrudes from the top of the door.

IMPROVEMENTS:
This design is a remix of the GraviTrax trap door "GraviTrax Kompatible Falltur" at https://www.thingiverse.com/thing:4620907, with 3 significant improvements:

1. Added a positive stop to hold door track in position when door closes
2. Tightened tolerances for a less sloppy operation when printed with high accuracy printers like Bambu Labs X1C.
3. Shortened the door so that it flips more easily, and in tighter spaces

ASSEMBLY:
Glue the tail into the door recess with the curve pointing away from the center of the door. Super glue (cryanoacrylate) works well. After ensuring the holes in the body are well cleared out, and the door pegs are well trimmed of any support material, insert 1 peg of the door into the body, then snap in the other peg.

USE:
Insert into base tile, raise the door, and let the 1st marble fall through the door. It will close the door as it falls, so all other marbles keep going straight through.

REQUIRES:
Use the trap door with the GraviTrax tunnels straight-through base tile, or with one of the base tiles listed below, which I have printed and recommend because they work with insert sizes that are the same size as GraviTrax base tiles. (There is nothing more confusing that having 2 slightly different sizes of part inserts...this is a hint for those of you who design 3D printed parts for GraviTrax.)

THESE BASE TILES WILL WORK WITH THIS TRAP DOOR:
https://www.thingiverse.com/thing:4723450
https://www.thingiverse.com/thing:5189236

This object was made in Tinkercad. Edit it online https://www.tinkercad.com/things/breBOOpccPS.

Bambulabs

Bambu X1C

Yes

Print with supports enabled (baseplate only). If you have a good slicer like Bambu Labs Slicer, the supports will peel off easily.