Indeed. Additionally the current route of the Cap through Pennsylvania has very little en route ridership, which has made the Cap a perennially weak train. In comparison the route of the Pennsylvanian has a lot of enroute ridership. Frankly the Sand Patch route is really not capable of supporting a second train in any meaningful way.
I am not sure why the Pennsylvanian is mentioned in the context of the Washington - Pittsburgh market since it does not currently form part of it.
I mentioned the Washington - Pittsburgh route in the same post as the Pennsylvanian mainly because they are both two routes that have to cross the Appalachians, and have very slow running as a result. You are probably correct that the Sand Patch route has a lot less potential in terms of intermediate stops, though I would think that a Baltimore/Washington - Cumberland service would do very well. There's certainly enough traffic on I-70 and I-68, and avoiding the traffic jams as an auto driver approaches Baltimore and Washington might make the slower train running along the Potomac more competitive.
West of Cumberland, there aren't many large towns, but there are a bunch of smaller towns (Myersdale, Garrett, Rockwood, etc.), and McKeesport actually had commuter rail service into Pittsburgh some years ago. Rockwood is actually close enough to serve as a station for Somerset, which is a county seat, and a reasonably decent-sized town.
The real problem with the route is that it's so slow. The current Capitol takes 8 hours to travel between Washington and Pittsburgh. You can drive it in 5 hours most of the time. (The highway distance is about 40 miles shorter than the rail distance, and even on the Pennsylvania Turnpike with its 1940's curves and grades, most cars can maintain the speed limit of 70 mph for nearly all the distance, sunless you get stuck behind a big rig grinding up some of the steep grades.) While it might be possible to speed things up, there are limits with the current track. The B&O ran a train in the 1950s and 1960s called the
Daylight Speedliner that did the Washington-Pittsburgh run in 6:20 (7 hours towards the end of the service). This was done using RDC's (i.e. DMUs), and it even had dining car service. Maybe with modern tilting trains, it could be run on a faster schedule, but who knows.
In short, the mountains really limit the possibilities of new corridor service unless somebody (the public, investors, who knows?) is willing to pay some really big bucks to upgrade the tracks across them by eliminating curves and such. It would involve cuts, viaducts, and maybe even a base tunnel or two. But if they could, consider a route like Washington to Cleveland at 450 miles, which is sort of close to the distance of Washington - Boston (457 miles). Washington- Boston can be done at 6-7 hours, even including slow running in Metro-North territory, plus the other NEC bottlenecks, whereas Washington - Cleveland is 11 hours. Washington - Pittsburgh is about 280 miles (240 miles by highway) but takes 8 hours on the Capitol. New York to Washington is 226 miles and can be done in 3:20 in a Northeast Regional. If they could speed up the running time across the mountains, there would be opportunities for fast day trains between New York, Philadelphia and Washington to Pittsburgh, Cleveland, Toldeo, and even Detroit. And even more, if they could get a Washington - Chicago service to average 70 mph, one could have an 11-hour day train between the east coast and Chicago to complement the overnight trains. (And with 70 mph average speeds, they could have 12 -13 hour NY - Chicago day service on the Lake Shore Limited and the old Broadway Limited route.