Three milepost references make for a jumbled mess at Point of Rocks:
1) the current Old Main Line Subdivision (BAC), 2) the Metropolitan Subdivision (BA),
3) the original B&O Main Line west of Point of Rocks. BAC and BA
are CSX's milepost prefixes. Warning: nitpicky railfan content ahead.
The BA prefix begins counting in Washington, DC, and extends some 42.8 milepost miles
to Point of Rocks where its mp number suddenly jumps so as to match the original Main Line's
mps as the tracks continue west. The sign at the end of the platform attempts
to explain, but instead adds to the confusion by mistakenly claiming BA 42.8 = BAC 68.4.
In reality, this location is 0.7 miles west of BAC's milepost 64, thus making it BAC 64.7.
By mentioning BAC, the sign implies westbound trains will be seeing BAC mileposts, but
that's not true. BAC is CSX's Old Main Line Subdivision whose eastern end point is in
Baltimore, and western end point is at the signals 0.2 mp in the distance.
The 68.4 value on the sign probably comes from the OML's original value prior to straightening
efforts that reduced its length, however, that's the wrong number. The sign's location is
about 0.3 miles east of BA mp 69. So, the sign would do better to read "BA mp 42.8 becomes BA
Though the track west from here was originally the B&O's Main Line from Baltimore, it became
part of the Metropolitan Branch after the B&O opened that branch in 1873.
Links: CSX BC Dispatcher refers to BAC 64.9 (mp3 audio),