[Scan-DC] What is the purpose of this sensor or antenna?

[Blair Thompson]

I go through only two underground Metrorail stations on a regular basis; Gallery Place and Judiciary Square. Last Monday, while I was on the Glenmont side of the upper (Red Line) platforms, I noticed some shiny new conduit. I followed it to a new (to me) device mounted at about the 2:30 (or 9:30, your choice) position on the wall. 

The device is housed in a gray box, larger than a cigar box, but smaller than a shoe box. It is positioned so that you could bounce a tennis ball off it, but it is beyond one's reach, maybe ten feet or so above the platform.
 
On Tuesday, I established that there are two at Judiciary, also on the Glenmont side. I saw one at Mcpherson Square, so they seem to be showing up around the stations, at least underground. I took a picture of the one at Judiciary Square that is nearer to the National Building Museum exit than to the MPD headquarters exit. I tried to send this post yesterday, with a picture attached. The submission was rejected, as it was over 40kb in size.
 
One person has guessed that it is a bomb detector. I have not noticed any increase in bars on my Kindle Fire, so it doesn't seem to be related to WiFi.
 
 Any guesses?