I think I heard the system in Mexico works well: https://en.wikipedia.org/wiki/Mexicable

The system in London works, but was not built in a very useful place. While it's a handy way to get from the O2 arena to the Excel convention centre (something people needed during the 2012 Olympics), it's very rare for a longer journey to be made quicker by connecting to it - most of the time you can go to a different station and change. While it is part of the London Oyster fares system, it costs extra to ride.