The only problem with using the 'old main' as I understand it - and this may not be correct, as it is something my Dad told me back in the late '60s or early '70s while watching trains at Point of Rocks - is that its curves are too sharp for today's longer cars. What makes me think the 'Old Man' was on to something is that in recent times I've only ever seen coal on it, and this is why CSX sends frieghts that are not hoppers south through DC via the Met. sub.
Its possible it couldn't originally accommodate 85 ft cars, but after damage due to Hurricane Agnes (1972), it took a while to rebuild the OML (and was almost abandoned) and my guess is that it may have been reconstructed with better geometry. Also, at some point (not sure if pre or post Agnes repairs) the majority of the line when from 2 track to 1 to allow for better clearance in tunnels, so that could have also helped in allowing longer cars. The line is still relatively busy, I hear train horns all the time from my house about 1 mile from the line.