CMR monorails can be spotted throughout the Capital Wasteland either on the track or on the ground (due to the track collapsing) in Fallout 3. The CMR monorails are not explosive, and do not serve any purpose except as scenery. The fact that there are so many wrecks and no monorail stations around the Capital Wasteland suggests that it was a high speed city to city line(s) within commonwealth boundaries. However, some monorail tracks can be seen with in the D.C. ruins (such as behind a pile of rubble near Reilly's Rangers HQ), so there maybe some stations in D.C. which are inaccessible by the player.
CMR monorails can be found in abandoned car fort, on the unmarked location monorail train wreckage, and throughout the Capital Wasteland. One west of Wheaton armory can be climbed on and inspected closely.