What if you made them really huge and mostly automated? Make the service be mostly self-service, like at an extended-stay hotel?

What's that supposed to illustrate? That a bomb in a crowded place can be harmful? Hmm, ok, better get the TSA to cordon off all malls, clubs, popular restaurants...

The thing about air travel is that there is a better argument for (some) security to prevent hijackings and bombs, both because planes are very fragile (there's a decent chance for a smallish bomb to result in the entire plane being lost and all passengers killed), and because they're usable as missiles against much larger targets (as illustrated by 9.11). Planes, in other words, have a somewhat unique ability to magnify the effect of a given terrorist attack.

None of that is the case for rail transport. For the most part, a train is simply a very crowded place, in a society that's absolutely chock-full of very crowded places. Indeed, the long-distance/regional trains most likely to receive TSA "attention" are much less crowded than local trains and subways which will almost certainly never be allowed to fall under TSA control for practical reasons.

At any rate, there aren’t any in Germany, even though two islamic terrorists actually targeted two German regional trains in 2006. They left suitcases with – luckily – non-functional bombs in both (http://en.wikipedia.org/wiki/2006_German_train_bombing_plot). That’s pretty much the only (attempted) islamic terror attack that ever happened in Germany†.

Bigger train stations usually have quite some police presence and there are (horribly racist) spot checks, but that’s all. (Usually the police want to check your ID card and then radio in the name to see whether there’s anything on your name. I’m guessing they are also trying to look for suspicious activity, but the most permanent reminder of the possibility of terrorist attacks are regular announcements to not leave your baggage unattended.)

Even when there was a lot of fear of terror attacks on trains or train stations being imminent a few years back, police presence (in bigger train stations) was merely bumped and you got to see some policewomen and men with submachine guns.

Everything would probably collapse in Germany if airport style checks were introduced in train stations. It’s just not realistic. The way trains are used doesn’t play nice with security checks. I mean, tons of people use trains in Germany to commute. Those trains are already plenty late in many major metropolitan areas, add security checks to that and you have a recipe for disaster. Plus: There are so many train stations, do you need checks in everyone of those? Even just including every high-speed train stop would probably result in many more train stations in Germany needing security equipment than there are airports in Germany.

But everyone can see that high-speed train stops are not enough. The terrorists in 2006 targeted regional trains! Those stop at train stations that are no more than a block of concrete next to the tracks. And those trains are plenty packed, often more so than high-speed trains.

Traveling by air has a different pace. It fits security checks better. There are few airports and airplanes don’t make dozens of stops god knows where.

The economic cost of bringing airport style security checks to trains easily outweighs any benefits even security fanatics might fantasize about. That’s why it doesn’t happen, at least not in countries with heavily used and dense train networks. (Now, countries with very sparse train networks that are not heavily used are a completely different story. Airport-style security checks do happens in those countries. Uzbekistan would be one example.)

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† In 2011 an Islamist shot and killed two US soldiers (wounded two more) on the way to Afghanistan who were in a bus in front of Frankfurt Airport (about to be transported to Ramstein Air Base).