Zimri
Well-Known Member
From our perspective as tunnel engineers the PED are there for ventilation, airflow and safety reasons. It was discussed on another thread that I can't now find how the tube acts as a giant piston on the older stations - I say the older stations as primarily they're a lot smaller in diameter than the new tunnels we're building and so there's no room for escape paths, or as much room for the air to flow around the train, a quick google turns up this image of a Northern Line train exiting a tunnel up near Hendon:
It's not ideal but it perhaps gives a good idea of the size of the trains and the tunnel lining on the older parts of the network. So going with that idea, and then adding the idea of retrofitting PED to the older parts of the network, Jubilee line included there just isn't the space to do it, and the costs will be massive. We're trying to upgrade Victoria Station at the moment without closing it and it's chaos, the same with the Crossrail stations where we're building new platforms and then linking them into existing construction.
The station platforms are obviously a larger diameter than the running tunnels, but not significantly larger and perhaps the biggest issue is that they simply weren't designed for the number of people using them now, or the numbers that are being envisaged for the next 5/10/15/20/25 years. You can't get into these stations and replace the cast iron linings easily, if at all without closing part of the platforms down or possibly the whole platform. That's not a viable proposal for many stations, it would simply cause too much disruption for all users, both at the closed stations and the stations that pick up the horde.
I'd also be surprised if you could significantly increase the loading on the old cast iron lining without further significant problems being encountered. We use a safety factor of somewhere around 1.6 these days (It differs for different things underground) but when these stations were designed the factor of safety might have been as low as 1.0 or 1.1 (anything over 1.0 is safe), you couldn't hang the PED from the crown of the tunnel similar to what we're doing here on Crossrail, and loading it all on the platform structure as they've done on the newer sections of the Jubilee line might require the whole platform to be rebuilt and potentially the lining to be replaced, an absolutely massive job.
I've gone back through the thread and i'm not totally sure what the question is, but in terms of PED it's not on all stations because at some it's not possible due to size limitations, it costs a fortune and if it only increases safety it's possibly not worth the investment, and now on Crossrail it's going to help ventilation, airflow AND increase safety at what are going to be some massive interchanges and stations.
Crispy has beaten me to it, but there's your tldr; version
).
I'm on a mission to lose some weight using the 220 stairs we have in and out of our tunnels at Liverpool Street, but I think i'm allowed this biscuit... 
