On Mon, 9 Oct 2017 01:06:42 -0700 (PDT), wrote:



Would it be possible to get a 2nd layer of seats in if carriages abandoned bogeys and implemented modern suspension with a resulting low floor level?

It's been done, but they weren't very successful:
http://en.wikipedia.org/wiki/SR_Class_4DD

The capacity increase is considerable, and they remained in service a long time, which suggests they did work well.

What was found that attempting to get more passengers through the same
amount of doors especially in crowded rush hour conditions took too
long which meant they had to stop longer at stations while passengers
from the upper level extracted or forced themselves in through those
already crammed in the lower level.
If a fleet had have been built then the end to end times of journeys
would have been longer as even 30 seconds delay over 15 to 20 stations
starts to accumulate.
They were always considered cramped and unpleasant by the passengers
and were well before the air conditioning age even though they had
some pressure ventilation , because non of the windows could be made
opening due to position and clearances the upper levels were very
stuffy and used reluctantly. Must have been hell with smoking being
allowed and with Lifebouy used on the weekly bath being about the only
protection against BO.
Squeezing a similar design now into the cramped British clearances may
at least get some air con but since the forties people have got a lot
bigger.

As you noted it was decided to provide longer trains instead.

As prototypes they did have quite a long life ,the mechanical bits
were common with a lot of other units so did not really need any
special parts or skills which helped with that.

G.Harman