I've found for now, diesels in IR have only 1 set exhaust pattern, but with dynamic exhaust effects it could make them a lot more realistic and interesting. For example when you first start up a locomotive in game there is only a small amount of clearish smoke which turns to lots of black smoke as throttle is increased. Now with dynamic exhaust you could have a lot more variation, such as lots of smoke, none at all, or anywhere in between. This could tie in well with the parts wear idea I've seen floating around a lot, as when parts such as pistons or turbochargers tend to wear out, the smoke produced would be a lot more and a lot darker.