It turns out that high speed trains are a lot more robust than planes: the failure mode is usually "soft", i.e. the train comes to a halt safely and damage is confined to one carriage (unlike an airliner). To inflict mass casualties (e.g. 7/7, Madrid) attackers target either busy subway systems at rush hour (7/7) or busy transport terminals with lots of people (Madrid). It's worth noting that both 7/7 and the Madrid bombings involved large suicide teams and multiple bombs -- but killed fewer people than a typical airliner bombing.
It turns out that high speed trains are a lot more robust than planes: the failure mode is usually "soft", i.e. the train comes to a halt safely and damage is confined to one carriage (unlike an airliner). To inflict mass casualties (e.g. 7/7, Madrid) attackers target either busy subway systems at rush hour (7/7) or busy transport terminals with lots of people (Madrid). It's worth noting that both 7/7 and the Madrid bombings involved large suicide teams and multiple bombs -- but killed fewer people than a typical airliner bombing.