Only tunnel that was bombed in recent times that I can think of was the Picadilly line. Madrid was just a normal commuter train wasn't it? If you want to cause maximum impact, put a bomb in a suitcase on a train going into a London terminal at peak time - you don't even need a suicide bomber, just have them get off a stop before, you'll have far more impact than anything a metal detector at St Pancras will catch. A bomb on a packed tube train (like the Russel Square one) will have far more impact than a bomb in the channel tunnel.
Seems that in bygone times, Italy suffered a fair bit from train bombings in tunnels (Italicus Express etc). I don't recall ever going through a metal detector to get a Trenitalia train.
I can think of more than one time the channel tunnel was closed due to accidental fires though, I'd have thought a car with a tank full of fuel would be far more concern than someone boarding a eurostar with a screwdriver.
Seems that in bygone times, Italy suffered a fair bit from train bombings in tunnels (Italicus Express etc). I don't recall ever going through a metal detector to get a Trenitalia train.
I can think of more than one time the channel tunnel was closed due to accidental fires though, I'd have thought a car with a tank full of fuel would be far more concern than someone boarding a eurostar with a screwdriver.