To improve the appearance of your layout you must move on from just a flat board and add hills and valleys. When the railways were built the engineers had to build bridges, viaducts, and as well as cutting and tunnels to ensure the railway track is flat or if on a gradient it was a gentle one.
Bridges, viaduct and embankment were required to fill in valleys and take the track over rivers and roads. Cuttings and tunnels were necessary to go through and area of high ground rather than going round it.
As our layout starts off on a flat board we (the modeller) have the advantage over the engineers in the past. They were adding the railway to the scenery while we are adding the scenery to the railway so the heavy work is much reduced.