I'll nominate London Overground/LOROL, 2007-2014. Started well with a general refresh and tidy up, and just kept getting better with new trains, new routes, then bigger trains, then more trains and bigger trains again! Threatened closure of the small stations at Rotherhithe and Wapping to allow the bigger trains averted by technology. Still some improvements going on, such as night service on the ELL section.
However, starting with the takeover of GE Lea valley services, all seems to have lost focus and more problems than solutions at present. Hopefully it will get back on path soon, and TBH the 'core' Overground is still fine, just a victim of it's own success and popularity.