Its hard to parse your question. I mean to say, I understand all your words individually, but strung together like that, they make no sense.
which element cools down when heated.
Umm ... all physical objects cool down when heating stops. No physical object cools down while its heated. Unless energy is spent to reverse the process. I can build a refrigerator with coolant, a compressor, an evaporator, but there's no element that just does that. Where would we find this element? Dug out of a mine? What would keep it from freezing? If there was water nearby, wouldn't it freeze forever? Even in the desert? How would we reach it? Would we be able to live nearby without freezing to death? Would such a world exist? Could humans live in such a world, and would it prevent them from developing a technological civilization?
can you help
I can try to help you understand, but I don't think the problem, as you've asked it, is solveable.
clue given : very soon, it is going to be used in the tunnel railway tech...
Why a clue? Is this a game you've made up? Or an invention or process you need and haven't thought out fully?