Andy wrote:

DfT have indicated that they are willing to accept less than 140
vehicles, if it involves use of existing stock. The planned 458/460
rebuild would provide 54 extra vehicles and the 456s an extra 48 to
give a total of 102. Making up the shortfall of 38 (about 27% of the
planned vehicles) would have to depend on new build. (This was all
covered in the February Modern Railways).

Yes I saw that - I folowed the text OK, though I thought Roger's latest
table was a little confusing, because he included the existing 458s - which
on the face of it shouldn't be there as they are already in current use -
but not the 456s.

With regards to running the trains at maximum length, in peak
direction, it would be easy to diagram the 10 car trains to be
arriving in Waterloo in the 08.00-08.59 'main' hour, with the 8 cars
running the other services and / or those on the less busy branches.

But the same NR table explains that by running ALL suburban trains at
maximum length (which apparently requires only 12 extra vehicles) 1,200
extra seats become available across the whole three hour peak.

However what is more relevant in context is that this provides no increase
at all between 0800 and 0859. That suggests all suburban trains that could
run as 8 car between 8 and 9 already do. A subsequent look at the Waterloo
CWNs also supports this view. (I also recall there are no single 455
diagrams at all on weekdays.)

Therefore as there are supposedly no additional paths available, nearly all
the suburban increases must be due to train lengthening, indeed the
figures are vastly different - 1,200 by running all trains at the current
maximum, but 19,200 by running all at 10 car.

I would say it is impossible/very expensive to convert the whole 455
fleet to 10 car operation (along with consistent power per unit when
running in shorter formations). Of course, in the future, 12 car will
be easy.

Which is about where we started - only able to lengthen half of the 455
operated services.

Paul S