The choice of the type of environment is very important and however depends, as for tracks, on the available space. Avoid the mountain too high and therefore little realistic: prefer small slopes and use eventually painted backgrounds with higher mountains. The scale used influences the landscape too: N scale is suitable to reproduce wide spaces capturing the attention, the HO scale instead, for its dimensions, sacrifices landscape in favour of trains (unless you have great room at your disposal). Imagine your tracks as if they were layed 'on the landscape' and not after it (as it happens in the reality): we must give the impression that the railway adapts to the landscape, with bends, trenches, earthwork and bridges. On the contrary landscape adapts to the railroad but this must be our little secret!!
The sharp bends must be hidden (generally by tunnels and mountains) as bends, bridges, trenches and tunnels must 'hidden' the real track development.