Watching the fun unfold from a safe distance, it looked like things were able to move around Radlett Jn at least, as a few trains seemed to shunt there to return north. Presumably that's what edwin_m was referring to? Whether that required the isolation to be shortened (I'd understood that the ECO could only isolate a relatively large area, with more control possible once there are chaps 'on the ground' to operate switches manually - or am I mistaken?), possibly only as a temporary measure, I don't know. Not much point trying to run south of Luton whilst the railway's full of trapped units waiting to shunt clear. The T3, when taken, seemed to extend as far as Radlett Jn, presumably for a valid reason but probably precluding any further shunts there. FCC eventually managed to get a decent service as far as St Albans though (reversing by a double shunt via the centre siding) which is about as good as they could hope to manage!