Originally Posted by Roger

gleff: I think the simple answer is because many European trains are the same, often combining single-country sectors within an international routeing.

Take Frankfurt-Milan, for example. This would combine intra-German, intra-Swiss and intra-Italian sectors as well as the end-to-end route. I used to take parts of the Paris-Istanbul train, which ran sectors inside France, Germany, Austria, Hungary, the former Yugoslavia and Greece IIRC. (I never managed the whole route, sadly.)