As ridiculous as it sounds, the title says it all. When I add Create Power Loader fabric to my modpack, some trains start spinning wildly, and all of them have bogies rotated 90° off axis.

This all was observed with power loader only installed on the client and not present on the server.

When I teleported away and returned, a few wagons / locomotives were turned 180°

It can also happen that trains spin wildly. This seems to often be caused by tp-ing a larger distance (~5k blocks) thought this has not proven reliable.

Client log without Power Loader: https://mclo.gs/toJvcd5, toJvcd5.txt
Client log with Power Loader: https://mclo.gs/JWCv588, JWCv588.txt

I think this has to be installed both on the server and client.

But just having a modpack with the mod and playing on a server that doesn't have it should not break all trains.

This is not a valid assumption to make for any non-client side mod. Crashes and glitches should be expected because the client is expecting the server to send data that doesn't exist. It is a natural consequence of using the mod in clearly unsupported ways.

What should have happened is that the server kicks you for not having a matching mod list, but this isn't implemented in fabric yet, so it is up to you to make sure your mods are compatible with the server.

I am well aware it is needed on both the server and client to function. But just having a modpack with the mod and playing on a server that doesn't have it should not break all trains.