B &O was (comparatively) weak long before the post WWII decline of the "Rust Belt." Not sure why-- factors MIGHT include
--trying to compete with NYC and PRR while being only half the size
--not reaching the New York-New jersey region on its own rails, and having a very much smaller metropolis at its eastern end
--having a long and roundabout route between Chicago and New York, two markets which were more important (relative to the total economic activity of the nation) in early 20th C than they are now
--having a worse crossing of the Alleghenies than PRR's, let alone the NYC's "Water Level Route" (Cranberry and 17 Mile grades are both over 2%, I think, and longer thanPRR's 1.8% climb from Altoona to Gallitzin)
B&O was controlled by the PRR for a time in early 20th C. In late 19th C I think B&O did more "rebating" (unadvertised discounts to big shippers) than PRR. This strikes me as something a more hard-up (not to say desperate) management would have resorted to (and the sort of thing that would make the PRR's management unwilling to allow the B&O to remain independent).