John wrote:
Cab signalling would not work for NLL at the current time because the
route is used by so many trains which are not fitted. The costs of
fitment would be very high.

ELLX would have been a good route to try it on with all new trains,
but I think they decided the technology was not mature enough when
they had to let the contracts.

That could apply to the core route, but the extensions over NR routes would
have then required dual fitted trains, for the same reasons as you point out
for the NLL.

Paul S