Older GE's and Alco's smoke due to turbo lag. As stated above, EMD's turbo's are geared to the engine to act as a supercharger until higher RPM where they free spool. 2 stroke diesels require positive intake pressure to run. 4 strokes can pull a vacuum and don't need the supercharging effect. As a result, when the governor adds fuel the turbos are not spinning fast enough (being driven by exhaust gas) it will result in incomplete combustion. As the exhaust gasses build, the turbos are able to force enough air into the cylinder to completely combust the amount of fuel the governor is dumping in.

Newer electronically injected units will add fuel in a more judicious manner following the boost curve so they do not overfuel and smoke.