How do you know they would be mainly standing? I'm not sure how many would get crammed into a carriage on the london underground, but I wouldn't say it would surpass 120
The suggested seating layout for the new EMUs to be utilised on the Auckland City Rail link features a mix of longitudinal and 2+2 seating. The chosen seating layout for the trains is still being consulted on, but the present mock-ups suggest a seated capacity of around 76 seats per carriage:
http://www.aucklandtransport.govt.n...rent-projects/Rail/Documents/mockup_three.pdf
This means that just over a third of the capacity of each carriage will be made up of standing passengers if they are intended to accomodate 120 passengers per carriage, which is a ratio that is pretty much standard across commuter EMU designs.
It's probably true that you can't accomodate 120 passengers in a single tube carriage, but the carriages of a tube train are considerably shorter than that of heavy rail stock: A six-carriage C-stock tube train is approximately 90 metres in length (15m carriage length), while the three carriage EMUs destined for New Zealand are likely to be around 70 metres long (23m carriage length).
Tube trains feature a "Metro" style interior that is optimised for standing passengers, so that the number of standing passengers compared to seated passengers is far, far higher than you would find on a standard commuter train. For example, each 3-car Auckland unit is expected to accomodate a total of 375 passengers, seated and standing. So a pair of units would allow for a maximum capacity of 750 passengers, and would be around 140 metres in length. Meanwhile, the new Bombardier S8 stock that is being introduced on some of the LU surface lines is 127.6 metres in length, and provides capacity for 1226 passengers, only 306 of which would be seated.