https://en.wikipedia.org/wiki/List_of_driverless_train_syste...
The length ranking seems to be something like:
Riyadh - 176 km
https://en.wikipedia.org/wiki/Riyadh_Metro
Dubai - 90km, first operation 2009
https://en.wikipedia.org/wiki/Dubai_Metro
London DLR - 38 km
https://en.wikipedia.org/wiki/Docklands_Light_Railway
Shanghai is probably in the running as it upgrades: