As a general rule shunters cos of their lo speed and mainly running in industrial areas tend to pick up grime from the wheels and it spreads evenly bottom to top with a heavyer dusting on top also they tend after a few years to become dended and rusty
Hi speed locos tend to have weathering running up and back from the front highlighted on raised egdes spread from rain etc
if i was doing yours without use of a airbrush i would choose rattle cans colour/s that most suited the enviroment it works in so lets say a cement works..... white and grey and try for base colours ( that normally need laquer to add gloss) shake cans well then spray through metal flour sieve standing well back so loco gets dusted (not sprayed) letting the colour build up in turn untill it looks right.
if high speed loco do the same but spray from front
use metal sieve as it can be washed out with celulose thinners so she wont notice


and it stops you applieing to much to quick and no large blobs
remember locos take weeks to gather dust and dirt so Take your time
Tony