The shunting wagon from Sim Nation has an opening tool box door linked to the Hand brake
I asked if is could be made independent but the answer came back this is the only way to activate for a wagon
The control blue print is for the brake
The animation for the brake has the door animation as well
Certainly works and the only opening door that I know off
You can apply the brake through the HUD on the loco before uncoupling
Easy as and an existing control point for a wagon
Check out the image
Hope this helps
Ausc