Creative input on creating realistic terrain
Creating realistic terrain is somewhat difficult...mountains were pushed up by forces in the earth, eons ago...rivers cut channels and gaps in the mountains...RR's usually followed the path of least gradient: criss crossing the "Rivers".
 
Laying track first, usually results in terrain that has rivers flowing uphill, and canyons that don't look like they were cut by rivers or glaciers. Most mountains in the US are in actuallity big, low rolling hills. Only the Swiss Alps, Canada, China, and Bryce Canyon...etc...etc... have jagged pointy peaks.