Busses and trains are not magically immune to explosives. Especially when they have literally zero security and you could easily bring an entire suitcase full of dynamite onto them. The fact is, there just aren't enough people interested in that who have the necessary skills to actually obtain explosives without alerting the FBI to their existence.
If you just want to blow something up there's nothing even all that special about "transportation". Planes are special, because they have an unusual characteristic; taking control of a plane gives you an extremely powerful weapon that you can point anywhere and deliver incredible destruction. There aren't very many other things that two or three people can do that will credibly take down a skyscraper.
As it gets harder to deliberately crash a plane and there's ever more drive-by-wire, though, that threat diminishes, and the airport threat diminishes with it. Hypothetically if planes were completely ground-controlled, there would basically be no reason for airport security at all... blowing up planes isn't that intrinsically interesting, really. There'd be a certain amount of inertia to the concept, but that's it. (Yes, I know it used to be done, but post-9/11 I can't help but think that terrorists would view that as a pathetic second-best effort now.)
(Though I'd appreciate some serious security around the ground control station, though. Taking over one plane is one thing... taking over "all the planes in the area" or even "all the planes in the sky" would be something else!)
Is the TSA really doing anything to prevent someone from taking control of a plane though?
Post-9/11, the cabin door is now shut and locked during most of the flight. I can't imagine a pilot or crew-member would open that door due to a terrorist threat, knowing what we do now (and that they would probably die anyway, if they opened the door)
The door has also been substantially reinforced. It's probably safe to bet that anyone who tries to break through will not be successful for a variety of reasons. :)
As I understand it, FAA regs require that there be two people in the cockpit at all times. So, the Germanwings tragedy would very likely not have happened in the US.
That would be true if at all times meant 100% all times. But I doubt that this rule exists and is assumed to exist to keep the pilot himself from causing harm. But rather to notice medical conditions as soon as possible. So from this psychological stance it is very likely that there will be moments of maybe no more than a minute of someone being alone in the cockpit - realistically.
Pre-9/11 airplane hijackings were a moderate inconvenience. At worst you wind up in Cuba or something and have to find a way to get home. Post-9/11, and by post-9/11 I mean literally hours after the towers were struck, passengers knew better than to comply with hijackers, which is why Flight 93 ditched into the ground.
there's also the fact that it's easier to blow up a plane. you need a big bomb to destroy a bus but only a small bomb to make a hole in the plane and kill everyone. and the plane has way more people on it than a bus.
A hole in a plane's fuselage or wing doesn't mean that everyone dies. You need a lot of power in order to take out a wing. Remember that wings on commercial jetliners are designed to eat an entire disintegrating jet engine in order to protect passengers in the fuselage.
Additionally, a jetliner's fuselage can take a VERY large hit at cruising altitudes and speeds [0] and still leave the plane flightworthy.
Planes are tougher (and their crews better prepared) than people think they are.
True, however I would imagine that an attack on an airport would hurt more than an attack on a bus or train depot; I'd wager that flights might get grounded until everything is sorted out. Nobody's going to shut down New York's Penn Station if something happens in Chicago or San Francisco.
Busses and trains are not magically immune to explosives. Especially when they have literally zero security and you could easily bring an entire suitcase full of dynamite onto them. The fact is, there just aren't enough people interested in that who have the necessary skills to actually obtain explosives without alerting the FBI to their existence.