Steamy !! As Alco38 points out, you can buy in many different flag paint schemes and then base your layout on THAT region. You can also decide FIRST on the region and then search out that division's paint-scheme.
Goodness knows, in HO-scale, you'll have a WIDE selection (I'm cursed with the smaller selection (but growing) of N-scale products) to chose from !!
MOST of the good model loco-producers (Altas, Athearn, Bachman, Walthers, etc) will also produce all their locos in an "undecorated" version.
Then with just a touch of spray paint, alittle masking tape, and some decals or dry-rub letters.... you can produce your OWN paint scheme and road-name. In model-RR parlance, it's called "free-lance" (as opposed to "prototype").
I've created 3 switchers using that technique for the "Stockton-Yamson Traction Company" to service the industries on my N-scale layout... my layout is based on current-day Stockton, California area with Union Pacific, ACE (Altamont Commuter Express), and SYTC !
DO invest the extra cash into a DCC system... it is SO much nicer to run that the old system and it's block-power wiring hassles !!
DO also buy a subscribtion to "Model Railroader" magazine, AND visit their website !!
Good Luck !!