Ok, I didn’t realise the physical version seems to be in germany only (at this point). Especially this version is the cheapest. Probably I have to thank Astragon. ^^
Nevertheless, normally you can find a “direct sale” on the dev’s page which gives you a steam key, but the dev’s get more out of it but directly bought via steam.
Btw Fun Fact — Train Fever is #1 at amazon germany .. in category “Adventures” (wh00t?)
http://www.amazon.de/gp/bestsellers/videogames/403296/ref=sr_bs_1
Cheers