In message , at 23:30:39 on
Fri, 2 Nov 2012, Jarle H Knudsen remarked:
A train is deemed full when all the seats are occupied. After that
people can stand in the aisles and vestibules, making it more than 100%
full.

The above for "Intercity and outer suburban" trains.

Is this also true for London Overground trains with longitudal seating?

It's a bit more complicated for "Metro" services (journey times
averaging less then 20 minutes), where traditionally an additional 35%
standing is allowed before the train is deemed "full". More recently
(trains constructed since 1999), that has been changed to 0.45m^2 of
floor per passenger.

For modern trains with designed-in standing space (like the Overground)
the capacity quota relates to "all seats full, plus three persons per
square metre standing".
--
Roland Perry