Will Duplo trains fit onto a regular 6 stud LEGO track?
For this experiment we're using the wheeled Duplo push and go motor block and red cabin from Set #10874.  

The wheel width of the motor is the same width as a non motor base.

  

So, for this train to stay on the six stud track, the wheels need to sit either on the inside of the tracks or the outside of the tracks.  

As the last picture shows, the wheels float on top of the tracks, which allows the train to slip off.  

If this red train represents the standard width of most Duplo trains, then we can conclude that a Duplo train will not fit on a six-stud gauge LEGO track and roll freely without derailing.

















UPDATE: Just getting our digital groove on.