There's a lot that goes into the emmisions of a Diesel engine. From a design standpoint, a few major things affect this: type of combustion chamber, direct or indirect injection, type of fuel system, injection pressure, turbocharging. For the maintenance end of the game it could be: clogged air filter, leaking nozzle(s), fuel system out of adjustent (either by use or sticky fingers) intake manifold leaks on a turbo engine, in need of tune up. For long term age issues, low compresion, burning engine oil, worn turbo / blower, fuel system wearing out, burned or damaged piston which could result from excessive nozzle leaks (I've seen holes burned through pistion crowns from faulty nozzle leaks or spray patterns. Sometimes it's bad fuel which makes smoke and can eventually claim the fuel system ($$$$).

Fortunately, there are only a few parallels to a steam locomotive - fuel, combustion chamber, "fuel system", and messing with said fuel system.