Some years ago, after ARB and the air district were handed their heads regarding *regulation* of locomotives in the LA Basin, they came up with some kind of agreement the railroads signed on that preferred use of cleaner equipment there. How that can be enforced for through trains, if it can be, is an open question, and I don't know whether it's still in effect. Could result in more newer GEs into LA, if it is still there. One visible result was the squad of gensets that arrived there for yard and local service earlier than in most places, with state/air district money helping with the purchase price in some cases. Could be that the agreement (if there was one and if it's still in force) was only for the LA Basin so trains into and through the SJV can use pretty much anything.