That's what's not happening - the tripcock was pushed back by the
raised trainstop but then sprang back to the vertical position rather
than latch up and allow the main line air to vent (thus operating the
pressure switch). Had the train been tripped at a faster speed, a good
'whack' would have ensured the tripcock was knocked back and up.

Seems odd that the Jubilee Line has not been forced to double-man
trains too, or indeed that HRMI haven't 'grounded' the whole 1995/96TS
as was done with the 1992TS after Chancery Lane.