After one terrorist attack on a train, all of the security restrictions will become just as cumbersome as air travel's.

Important difference: you can't fly a train into a building.

Edit: easily :)

Eurostar checks are very tight and even on busy days it won't take over an hour of queuing at Gare du Nord. Usually there aren't so many trains to board so it's much easier to organize. I suffered a whole station lock-down on a busy Sunday night due to a large piece of luggage left near the security checkpoint. It delayed my trip only 2 hours, including the delays of not being on time for the cleared route. Another thing is trains don't usually have busy schedule of airplanes so they usually wait for people if something like that happens.

Also there's no fuel around to use as explosive material.

Spain had its share of train terrorist attacks, and at least in the rest of the Europe you can't tell. I don't know about inside Spain, though, but I imagine it's still not much difference.

True. But we tend to overreact here in the US.

In Spain they X-ray your luggage before you can get to the platform for most long distance trains.