by Sand Box John
OK. We now know what the purpose of the cabinet is. What I find odd, is that it is on the platform. WMATA typically places the evacuation cart cabinets on the ground beyond the platform.

Springfield-Franconia, north end ancillary area
The only places where you will find evacuation cart on subway station platforms is at stations where there is no open space at track level in the tunnels

Union Station, south end
Springfield-Franconia, north end ancillary area
The only places where you will find evacuation cart on subway station platforms is at stations where there is no open space at track level in the tunnels
Union Station, south end
John in the sand box of Maryland's eastern shore.