How likely is an ATO-failure and what is the procedure?

[100andthirty]

I remember doing the testing of the 96 stock on the jubilee line extension. You had a little leeway in stopping at the PED doors. It was tedious getting the trains in and out of Stratford Market Depot at the start as we used loco's to dead drag them in and out.
The thinking of not putting ATO on the extension at the start is that at the time they didn't want 2 different systems working together, not like now with other projects.

The Jubilee line extension was planned to have ATO and the trains retained their ATO equipment to drive the PEDs. It wasn't implemented because it was an new moving block system that didn't work. The conventional signalling was designed and installed in record time after the plug was pulled.