As a commuter who rides the train that feeds Fenway, I can say that's a major detriment. Nothing like 30 minutes in a packed train with sweaty drunk fans to end your day in the office 2-3 times a week.
Care to cite a source? It was my understanding that it in fact took a considerable force to bring a plane down, unless you happened to detonate inside the cockpit. Cabin decompression would at best kill a couple passengers and force an emergency landing.
And while a plane can be used as a weapon more easily than a train, I don't see how liquids would be the controlling factor in such a takeover.