Iain,
I saw 3rdrail answered on another forum. He's 100% right; The Keystone had an article about that very subject. The PRRT &HS can hook you up. Remember, this is The Keystone, which is the prototype magazine; not to be confused with The Keystone Modeler, the free online e-zine about modeling the PRR.
I don't remember which issue had the article though.
Modeling the Rio Grande Southern First District circa 1938-1946 in HOn3.