Project Zomboid base BaseVehicle.addPointConstraint() force-breaks existing constraints and chain state in MP. To keep 1 -> 2 while attaching 2 -> 3 (and preserve movement replication), Landtrain includes a Java class override:

client install: zombie/vehicles/BaseVehicle.class
dedicated install: java/zombie/vehicles/BaseVehicle.class
helper class (both): LandtrainConstraintAuthHelper.class in the same zombie/vehicles folder

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

Apply patch to game

Patch your local client install:

.\tools\patch-game-basevehicle.ps1

Patch the dedicated server install:

.\tools\patch-game-basevehicle.ps1 -GameRoot "D:\SteamLibrary\steamapps\common\Project Zomboid Dedicated Server"

Ensure both mods are enabled:
- \hrsys_towbars
- \hrsys_landtrain

MP requirement

For multiplayer, every connecting client and the dedicated server must run the patched class. If only one side is patched, towing links can desync with no explicit Lua error.

Restore vanilla class

.\tools\restore-game-basevehicle.ps1

To restore dedicated server too:

.\tools\restore-game-basevehicle.ps1 -GameRoot "D:\SteamLibrary\steamapps\common\Project Zomboid Dedicated Server"