http://news.bbc.co.uk/1/hi/england/london/10293456.stm

More pay requested because of increased workload and responsibility
following three car introduction.

Paul S

So is this the reason we don't get any three car trains at weekends
(even when the Jubilee line isn't running)? We've suffered years of
disruption getting ready for three car trains and more often than not
I still have to stand at weekends.