Had the Train Tunnels mod installed, tried it once - didn't work into my map so, abandoned. Did not deactivate mod and I started noticing trains randomly slowing to about half speed compared to other identical trains. This also happened to any train the player rode in.

Finally the game crashed and the error message said it was caused by the Train Tunnels mod. Deactivated the mod and trains sped up again. All now seems ok. Weird.

Slow trains is intended atm to reduce teleportation failures (for now). But that only happens if you have at least one tunnel.

Would still have been interested in the error message.