I'm not sure how deep you want to go, but in a crude nutshell, the CV began as an independent local enterprise in the late 1830s, running from Harrisburg to Chambersburg, PA, via Shippensburg and Carlisle; by 1841 -- and originally under independent auspices -- the route was extended to Hagerstown, Md., through Greencastle. In the late 19th century it was extended further from Hagerstown to Winchester, VA via Martinsburg. The CV came under PRR control in 1860, although the Pennsy kept it as a legally separate corporation until it was fully merged in 1919. Although for all practical purposes it was a PRR branch by the time of the Civil War, until the 1919 merger the CV had its own minority stockholders and was locally managed from headquarters in Chambersburg (but with dictates from Philadelphia), and had its own locomotives and rolling stock -- most of which, of course, were built to PRR designs and were numbered in specific number blocks the PRR reserved for CV equipment. The CV had its own shops (which, last I knew, still existed) in Chamberburg. (Also, last I knew, two of the CV's three gerations of stations in Chamberburg still survived.) As originally built, the CV ran down the main streets of such towns as Carlisle, Chambersburg, Shippensburg, and Greencastle, and in the case of Greencastle, the original early 19th century station building still stands. Bypasses and track elevation came in the early 20th century.
The completion of the N&W's Shenandoah Valley RR between Roanoke and Hagerstown in 1882 turned the CV from an essentially local line to an alternate link between the Northeast and South through the Hagerstown gateway. This relationship strengthened after the Pennsy got control of the N&W, particularly as the chief outlet for N&W coal destined for Northeastern points. Now, of course, the CV route is Norfolk Southern's primarily link between its ex-N&W and Southern Ry. lines and its ex-PRR/RDG/LV lines in the Northeast.
The Hagerstown-Winchester segment spent most of its life as a secondary line, although during the Pennsy's control of the B&O in the early 1900s, B&O coal was routed east over the CV through a connection at Martinsburg, and the Martinsburg-Hagerstown section was upgraded. (A later and more convenient connection was built at Cumbo, outside Martinsburg.) This movement continued until 1927, when B&O rerouted the CV coal over its already-established Cherry Run/WM/Shippensburg/RDG route. The Hagerstown-Winchester line survives under Winchester & Western operation.
As an aside, the Cumberland Valley's 1851 4-2-0 "Pioneer," now owned by the Smithsonian, is at the B&O Museum being readied for display there.
