Peter Masson wrote:

Perhaps the Victoria Line needs to be extended into the open air, at
least so the piston effect of trains can add ventilation into the
tunnels, and ideally so that the next generation (49 stock) can have
aircon that takes heat out of the system and dumps it in the open
air. Take over Chingford from National Rail? Quadruple Northumberland
Park to Cheshunt, with a extended Victoria Line running the local
service?

A few years ago, a significant proportion of trains went to Northumberland
Park anyway - is that still true? I always wondered why - maybe it is to
cool the line.