"Terrorism" is the act of instilling terror in people. Go look at the picture in that blog entry, and tell me: do you think that guy is making the passengers less terrified, or more terrified? (And don't go telling me, if you're a middle-class white person, that he can't be causing anyone any serious fear just because you wouldn't feel any serious fear of him.)
Don't get me wrong: I think there are plenty of security measures to defend against terrorists that are extremely good ideas, and extremely important to undertake. And even the very bad defensive measures rarely, if ever, horrify me nearly as much as the very bad offensive measures like mass-murdering Iraqi and Afghan civilians.
But really. The chance of murder weapons being on U.S. passenger trains is a very bad thing, but putting murder weapons definitely on U.S. passenger trains is not necessarily an improvement, especially not when they're in the hands of non-cops who probably have little or no training, much less experience, in how to avoid becoming trigger-happy at the slightest fright. Furthermore, even if we can trust this guy to use his gun safely, couldn't he be issued one a little smaller and instructed to keep it holstered or possibly out of sight entirely and draw it only if he sees an actual problem, to avoid scaring passengers needlessly? Issuing him a much larger gun than police officers normally carry and having him march around with it drawn at all times smells distinctly of those in charge wanting to increase people's terror.
And yes, I know the people who instituted this would probably say the idea is to terrify terrorists into not terrorizing. But terrifying everyone for the sake of hoping to terrify some possible terrorists along with them is like the U.S.'s policy in Iraq of just throwing everyone they found in a whole geographic region into Abu Ghraib in the hope that they'd get a few guilty people along with the many innocent ones.