Ghost Train

Sorry to have not posted in a while. I've just gotten back from job-related meetings in chicago.
I managed to get on an earlier plane to Washington (DCA) and decided to take the metro home. I got off the yellow line at archives to grab the green line. i like to change trains at less hectic stops all other things being equal. when i got off there was another train in the tunnel behind the one is got off of. the display monitor said this:

1 --
GR 3 Greenbelt
YL 7 Mt Vernon Sq

It is not unusual to have random quirks in the monitors. They work pretty well most of the time, and it's neat to know when your train will arrive, but the system is not perfect.

I was hopeful that the tunneled train was in fact a green lane that would serve as my chariot to columbia heights.

It pulled into the station. All the lights were off. Must be out of commission, and is being sent to the yard for the night.

It stopped. Those trains don't stop. what is going on.

All the windows were black and i couldn't see inside.

Then a guy becomes visible in the last car. He is wearing body armor of some sort and carrying
a very intense and scary submachine gun (looked kind alike an MP5 but i thought those were primarily carried in europe).

What was a blacked out ghost train doing with heavily armed guards?

A large shiny metal cart about 6 feet tall was wheeled out of the train and another was wheeled in. A few seconds later the ghost train left. What was it's mission?

My best guess: it is the money train onto which the daily cash reserves collected in the machines are loaded to be sent to a more secure area. Either way, i'll be extra careful next time i see a two car train pull through the station.