It's not a common sight, but Metro operates trains that solely collect and transport cold, hard cash. They are protected by heavily armed guards. If you've never seen one, or don't know about them, it can be somewhat startling when one pulls into a station. From reader Joseph:
About three weeks ago, I was trying to get on an Orange Line train for Vienna at Metro Center at about 10:30 p.m. As I was going down the escalator to the platform, a "no passengers" train pulled up. The doors opened, and to my bewilderment a uniformed man stepped out of the first car brandishing an enormous shotgun. Since he was holding it up, and I was elevated on the escalator, it was pointing directly at me.
I yelled out in shock, and he looked up and saw that he was pointing the gun at me, but he calmly and expressionlessly continued to while two other uniformed guys moved a box off the platform and onto the train.
Still eying me without emotion, he stepped back onto the train, and it left.
Prior to him stepping out, there was no announcement that any kind of police would be getting out. So I had no way of anticipating, right before I was threatened with a deadly weapon, that he wouldn't be a criminal or madman.
And yet he didn't hesitate to scare the shit out of me for no reason.
By the time it was all over, I realized that he was some kind of official, but still, it's inconsiderate.
Furthermore, this is America. Just because you work for Metro, it's not an excuse to act like the Stasi or something.