Which terrorists are you worried about? If al Qaeda blows up a pressurised train tunnel, then yes they can kill lots of people, heap a big cash cost on the tun

Which terrorists are you worried about? If al Qaeda blows up a pressurised train tunnel, then yes they can kill lots of people, heap a big cash cost on the tunnel operator... but they don't get any impressive pictures to put on TV. If the IRA/ETA/etc blows up a pressurised train tunnel, they impose a big cash cost on their target, plus cause lots of disruption... but in terms of effort/risk/reward it's probably better for them to blow up conventional rail/road infrastructure in London/Madrid/wherever.