In uk.railway NY wrote:
Timetables are fine but if you want to catch the next service to HW and are
travelling from (for example) Trafalgar Square it is difficult to judge how
long it will take you by each route on the Underground and therefore which
mainline station you should had for.

As already discussed, this happens for KX/Liverpool St to Cambridge. The
journey time difference is such that it isn't real competition: if you're in
about a 15 min tube radius of Liverpool St or further east then that route
can swing it, otherwise it's almost always faster to go to KX, except in
case of disruption. The arithmetic varies a little bit during the day (in
the peaks KX/LST are about evens, off-peak KX wins, late evening both are
slower but KX still wins) but not enough to sway it. It can also vary if
you want to do Tottenham Hale (all LST trains) or Finsbury Park (KX
semi-fasts) rather than the terminus, which can work out depending on your
start point.

The frequency, spacing and journey time to get between the two is such that
if you miss a train, it's still quickest to stay put and wait for the next
one.

Theo