Cities like London and Madrid actually have had bombs on trains. It's not a "security threat" to buildings above. What is a security threat to infrastructure a

Cities like London and Madrid actually have had bombs on trains. It's not a "security threat" to buildings above. What is a security threat to infrastructure are roads, where you can drive massive vehicles into people, transport and explode bombs, etc. The Manchester bombing for example was a truck exploding parked on a public road.