Minecraft Forge 1.16.4, Version 35.1.13
Create Version 0.3b (Dec 22nd)
The second a train is turned on in my server, it seems to have a random chance to kick different people.

For example , lets say there's people A, B, C, and D. First time A turns on the train, everything works perfectly fine. About an hour later, A turns it on again, A is kicked. The others turn it off so A can rejoin. After resetting the server, A tests it again, but this time players B and D are kicked. Turning it off lets them rejoin. Later, C turns on their own train alone very far away, where C and B are kicked. D attempts to get near it to turn it off, when they are also kicked. The owner of the server then has to open the world in singleplayer in order to turn off the train so then everyone can join.
This example has happened multiple times in the last few days. It sucks because me and a few others were planning on building a huge train system on our server.

This bug is a known issue, many people are experiencing right now. A patch for it will hopefully be out soon!

While the exact exception is different, this issue appears to be manifesting from the same root bug as #579 .

This was addressed in 0.3c, please re-open if this is still an issue.