I know that you were talking about individual commutes here. The city of Oslo, Norway was considering having a shared trunk in one corner, where the metro, the streetcar, and the intercity rail all shared a stretch of track. I believe it was to make efficient use of existing right-of-way. I think that there were problems due to electrification (750 V vs 15 kV) and other political problems.

There is a different section in Oslo where the streetcar and metro share a stretch of track using a clever interlocking.