In reply to Dave Garnett:
> Aren't there quite a lot of low tunnels, bridges, stations, overhead power lines and gantries and stuff?
Yes there are, but in Switzerland they had mountains to go through as well.
> Part of the problem with being ahead of the game in Victorian times is that we now have a lot of very old infrastructure. I imagine the cost of replacing all this would be huge and, in some cases, undesirable for environmental and cultural reasons.
The cost would be huge but, you double your capacity at a stroke and that capacity is there forever. You can still only fit so many trains on a line making them faster only gives a marginal extra capacity, and it's still expensive.
At the very least we should be designing new tack, with the double decker capacity built in.