Is anyone building high speed lines in twin track tunnels? Surely the airflow from the two trains passing each other would be horrendous at that kind of speed?
In Germany this has been done for relatively short (~8km) long tunnels.
The air flow from trains passing is not too problematic because your tunnel tends to have more free volume than a comparable singel track tunnel (for obvious reasons!), and because the pressure wave doesn't have time to build into anything nasty, because the trains are after all only 400m long.
This is why bridges don't require pressure hoods like they are proposing for tunnels on HS2 - the effect only becoems significant as your length of time spent in the pressure wave generating situation increases.
Since the trains will have passed each other in something like 2 seconds, nothing too terrible can occur, meanwhile the free volume of the tunnel available to each train being larger actually reduces the pressure effects relative to a single track tunnel the rest of the time.
You also mention having a second tunnel for emergencies, surely it’s far more efficient to have two smaller running tunnels interconnected so you can escape through to the other tunnel if required?
Well potentially but in the UK situation, probably not.
We will be running most of our tunnels at relatively shallow depths, or at least close tot he surface in some direction (in places like the pennines you could imagine running the tunnel inside a valley wall with near horizontal escape adits) so escape shafts become more economic than the second bore.
You would only have to provide an emergency tunnel bore over the section of the route that requires them, without altering the profile of the running tunnel at all.
There is also talk of allowing tunnels to be a single bore but separated from their escape passages by suitably fireproof safety walls.
But I don't know of anyone who has got that through a risk assesment yet - after all tunnels have long lead times.
It is worth noting that
various EU studies put the cost index of a double running line tunnel at 220, whereas calculations based on the options suggest a double tunnel with secondary escape bore would be only 190.
If you can use 1000m spaced access shafts the cost of the tunnel falls drastically to only 130!
EDIT:
Apparently the Bologna-Firenze high speed railway has all its tunnels built into a single operational bore.
Also apparently something like 95% of it's length is in tunnels......