British architectural firm NBB​J has an intriguing idea for next generation public transport: moving sidewalks [popularmechanics.com]. The technology, of course, already exists, and is more associated with "trying to get around airports," but they have a fully envisioned way to keep it brisk: fast and slow lanes.
The fastest lane would move at 15 MPH, with passengers either lazily coming along for the ride or adding in their own 3 MPH walk to drive up that top speed. NBBJ even believes this could move them as fast as a train would, all stops considered. The concept also takes advantage of existing subway architecture, putting the entire thing underground and turning it into a sort of pedestrian concourse with small shops and stands.
Moving walkways debuted at the World Columbian Exposition [wikipedia.org] of 1893 in Chicago, Illinois.