CMR

CMR was a pre-war High speed Railway/Monorail service which ran on elevated tracks.

A number of the Railway/Monorail Trains can spotted throughout the Capital Wasteland either on the track or on the ground (due to the track collapsing). The trains however are not explosive and do not show any purpose except for scenery means.

Behind the scenes
What the initials stand for is unknown, however, it is most likely to be "Capital Mono-Rail".