Well, to response to my own post (unfortunatly no response from the creator so far)...I think I found a solution. First I turned the mod from a Steam-mod into a local mod and gave it the name "NS PTT post_1".

Then I copied the "hbi1_v3" folder from the game archive, wich can be found in "mesh.zip/mesh/vehicle/waggon" and I placed that folder in the userdata-folder from Steam "1066780\local\mods\NS PTT post_1\res\models\mesh\vehicle\waggon"

Then I copied the files from the "hbi1" folder to the "hbi1_v3" folder and deleted the "hbi1" folder.

In the userdate folder from steam in "1066780\local\mods\NS PTT post_1\res\models\model\vehicle\waggon" you find a MDL.file wich is called "LexyGone_PTT_wagon". You can open this file with notepad.

In there you will find four lines:

mesh = "vehicle/waggon/hbi1/lod_0_hbi.msh"
mesh = "vehicle/waggon/hbi1/lod_0_w1.msh"
mesh = "vehicle/waggon/hbi1/lod_0_w1.msh"
axles = { "vehicle/waggon/hbi1/lod_0_w1.msh"

I changed these lines to:

mesh = "vehicle/waggon/hbi1_v3/lod_0_hbi.msh"
mesh = "vehicle/waggon/hbi1_v3/lod_0_w1.msh"
mesh = "vehicle/waggon/hbi1_v3/lod_0_w1.msh"
axles = { "vehicle/waggon/hbi1_v3/lod_0_w1.msh"

This worked for me.

I have to mention that the original file from Steam worked when I didn't used any other mods but the PTT Post Wagon and also in older save files. In my new games, were I use 400+ mods it worked with the method I described here. I think the original mod is not working with some other mods that I use, but I don't know wich one.

Anyway, here's the result:

https://gtm.steamproxy.vip/profiles/76561198302426969/screenshot/2497883033909191854/