Project Zomboid base BaseVehicle.addPointConstraint() force-breaks existing constraints. To keep 1 -> 2 while attaching 2 -> 3, Landtrain includes a Java class override:

zombie/vehicles/BaseVehicle.class

This is the same override pattern used by mods like Realistic Car Physics (manual zombie folder copy).

Apply patch to game

Run:

.\tools\patch-game-basevehicle.ps1

Ensure both mods are enabled:
- hrsys_towbars
- hrsys_landtrain

Restore vanilla class

.\tools\restore-game-basevehicle.ps1