It would be awesome if you could use the hearth on a create contraptions, like just imagine having a cooled train moving through the nether and not overheating.

This is similar in concept to #162. While it would be very cool, unfortunately in a Create contraption all of the blocks are "fake" blocks. They have a hitbox and render, but that's about it. The hearth in this scenario would be disabled, so it wouldn't be possible for it to warm you up while moving