I´m sorry to hear, that you cannot enjoy Train Fever as it was intended.
We took note of your stability problems some time ago and tried to reproduce and analyses it on our hardware. As you correctly stated, not the game freezes, but the system itself. Therefore we got in touch with Apple and fielded a bug report, but are since waiting for solution from their side.
In addition to that the Mac book Air 2013 does run on an Intel graphics chip, which is unfortunately not officially supported by the game.
Please be assured that we are trying our best to get the game working properly for everybody.