I can't stick to anything. I even have different coasts! UP, BNSF, Santa Fe, Southern Pacific, BN, NY Central, New Haven, Pennsy, Penn Central, Conrail, NS, CSX, Chessie and CN are in the fleet off the top of my head. And don't get me started on eras....
US stuff is saved by power running through from one railroad to another and I like variety. When I do run stuff I keep it to locos that could feasibly be run together and stock to match rather than say running my NY Central 20th Century Limited next to a trio of ACes and Gevos.
I do subscribe to the 'it's my railway I'll run what I like' philosophy. At the end of the day do what suits you. I'd love to be more disciplined and focused but I've resigned myself to the fact that it will never happen!