I have a small kid and a bunch of LEGO Duplo train track. (Like what's in this set.) Would "grown up" sets, like the Emerald Night or Maersk trains, work on that track? They look pretty close (the wheels are a little more than 4 studs apart) but I'd love to hear a definite yes or no from someone who owns both.
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.
It depends on your goal, as Jason said. He illustrated pretty well why out of the package lego trains don't run on duplo track. However, if you just want something for your kids to dink around with, it is possible to build a wheeled train that will drive on duplo track without using a duplo train as it's base. The trick is to use monster truck tires or "offroad" style tires, and a 2x4 axle that is flat. If your track is curved and not all straight, you also need your tires to articulate on swivel hinges.
Here's something I rigged up for my son from only parts I already have:
It worked well on a fully circular track, though it would catch a little if I made a squiggle track. I think the smaller off-road tires would have worked better but I could only find one four stud wide axle in that size.