Two points.
Firstly, LT trains work in the same way and I am not aware that they have
had problems.
Secondly, Once a tripcock is struck it remains in the up position until
it
is reset when the unit is uncoupled so the rear unit tripcock will only be
hit once per period of time that the unit is coupled as the non leading
unit.

The only difference being that the LUL trains will get strike the first
signal they encouter leaving the depot at 10mph whilst the Chiltern units
similiar first experience will be a signal at 75mph!

Andyh