Do regular Lego trains fit onto Duplo tracks?
The answer is kind of.

I own the Emerald Night and a bunch of Thomas Duplo track.  The wheelbase of the Emerald Night is wide enough that the "rails" fit inside the wheels.  In fact, the passenger car fits perfectly on the track.  Unfortunately, neither the engine nor the tender fit as well, due to the fact that both have pieces on the bottom that are too low to allow them to sit properly with the rails inside the wheels.

This means that the engine will not move on its own.  In fact, I have my doubts it would move well even if the wheels reached the track, as it would require the flanges of the wheels to have enough traction to pull the train.