https://paste.fedoraproject.org/paste/nClp0N8lnHN5m6BVXoX1pQ

All that line is doing is calling:
world.getEntities(EntityCoupleableRollingStock.class, ...)

It looks like some other mod is doing bad things go "world.loadedEntityList" in another thread

Please re-open, it happened again on a fresh server instance with no players running chunk pregen-

https://paste.fedoraproject.org/paste/8XbwPSuAKtEDUuEZ0qKNOQ

That is a different crash, but I'll re-open anyways

Ok that line is calling:
"List entities = world.getEntities(EntityCoupleableRollingStock.class, EntitySelectors.IS_ALIVE);"

On the main thread in a tick handler. Some other mod you have is messing with the loaded entity list in a different thread.