Looks like they've quietly been fiddling with steam loco simulation - the fusible plugs melting is a sign the water level in the boiler has dropped dangerously low, and previously at least water consumption was completely unsimulated...
I don't think it's a new message necessarily, a steam engine would definitely overheat without a tender
No, the tender by itself does not stop the locomotive from "overheating". Its supply of water does. If water is supplied from another source, or the locomotive is not operated for long enough to run out of water in the boiler, there is no issue. At present it is completely impossible to give a steam locomotive water in TSW.
