If its a realistic depiction of a railway you're after then nothing spoils the look more than clean and new models.
In real life, railways and the engines, coaches and wagons are dirty and dusty , track and buildings are worn and weathered and this video shows you don't have to be a fine artist to create that realistic look on your model railway.
An airbrush is very useful but not entirely necessary as you will see various techniques including dry brushing so don't worry about needing expensive equipment.
Take your modelling up to the next level and enjoy this indepth and high quality video tutorial.