I've ridden from Pittsburgh to Cumberland twice. I've never done the reverse. Pittsburgh to Cumberland is a very gentle (to my mind) uphill for 120 miles or so, getting slightly steeper towards the end, but always very manageable. The you loose all the elevation you gained and then some over the next 20 to 30 miles. So to reverse the trip, you finish virtually all of you climbing in the first 30 miles, but it's a lot steeper. I've never done it. The people I saw going fully loaded the other way didn't look too happy, but it's still railroad bed, so not super steep, and you finish it all at once. Not my preference, but, again, I've never tried it.
I've taken the Amtrak from Cumberland to DC. It leaves in the AM, but not super early, and, yes, it was about an hour late. But that trip was all done at reasonable, daylight hours and might be preferable to leaving Pittsburgh at 5am or getting there at 12am (or whatever time the train comes in. I can't remember.) But I don't know your travel details. Maybe you need to roundtrip to Pittsburgh. The other time I did it, we left a car in Cumberland, took a shuttle to Pittsburgh, and returned to the car over the course of a few days. Can't say the exact price of the shuttle because it was bundled with our lodging through Noble Invention bike tours.