I believe there are power supply issues as to why they only run 3 car units
to Watford.

Loopy

Are they actually planning to increase the length of trains, where will
the extra trains come from, also for increased frequency. Incredible
for a line that 30 years ago BR would hve quite happily confined to the
waste dump (passenger services anyway). I have always thought the 3 car
trains from Euston to Watford Junc needed extra carriages when you see
the overcrowding so would have thought that this should take priority
over the NLL.
Kevin