I've ran into a peculiar bug when dealing with conveyor rails - their content doesn't exchange heat with the environment if the content is in the miligram range.
I have a setup like in the screenshot - a metal volcano tamer that feeds into a conveyor rail with a temperature sensor that activates the exit from the system (the bottom chute is disconnected). When it works as intended it's really neat - the stuff on the rails exchanges heat with the surrounding steam, cools everything down and then you can ship the metal out without having to do multiple passes through a chute and auto loader. You run into problems when you're dealing with live volcanoes though - they drop their contents in small batches, so the auto sweeper loads as little as a few miligrams of metal to load. This causes the conveyor rails to move that around and probably due to optimisation such small quantities of materials don't exchange heat with the surrounding and you end up with 700C gold sitting in 120C steam and not budging a degree. You also get some empty packets of shipments on the conveyor rails, but that's some other bug it seems.
It would be nice to perhaps force the rail content to exchange heat with the surroundings no matter how small it is, otherwise temperature sensors for the rails may cause inaccurate readings that grind the rail to a halt...
Open up the save and check the active gold volcano tamer - if you have a tiny packet of metal being shipped it will halt, but will work perfectly with big chunks of metal.
- Set up a conveyor rail with a temperature sensor
- Load up 1mg of gold at 700C in a steam room
- Move the gold on the rail - it will stay really hot despite the room being way colder