Oh, sure, Cleveland-Chicago is definitely a possibility. Also Toledo-Pittsburgh. I was referring to the city pairs that involve crossing the mountains -- New York -PIttsburgh or Cleveland, Philadelphia-Harrisburg-Pittsburgh, Washington-Pittsburgh, etc. Going from New York to Cleveland by the Water Level Route doesn't involve crossing the Mountains, but it is a 150 mile detour compared to driving across I-80. In fact, I'm curious about how frieght is competitive with trucks on these routes, considering that the rail routes are a lot longer than the highway routes. (I'm thinking of stuff like all those double stack containers and car carriers, not the bulk coal trains.)
Now, back to passengers. Again, this is overlapping corridors. I've already explained the Lake Shore Limited.
On the Broadway Limited route, Philadelphia-Pittsburgh seems to be viable for train service despite the slow route through the mountains -- isn't that interesting? Perhaps this is because of the lack of bus service which you mention, and the decline in air service. Pittsburgh-intermediate points-Chicago is certainly viable. The decline in the alternative services is a key point here: the bus service from points in between NY and Chicago to either end is getting terrible, while the air service has dropped a lot from its former levels.
Now, there is a lot of endpoint-to-middle traffic to capture on these routes. NYC is the biggest metro area in the world and Chicago is the second-biggest -- so they attract travellers from further away than comfortable driving distances. But they're still within plausible train distance of all the points along the route (single-overnight at worst). When air service declines in frequency and quality, the result is burgeoning demand for train service. Syracuse-Chicago is one of the most popular city pairs on the Lake Shore Limited. Detroit-NYC is the most-requested city pair which Amtrak doesn't serve, as of a few years ago.
The third NYC-Chicago route which should be operated is the route through Canada, and this is partly because NYC-Detroit is a high-demand route, and partly because Detroit-Upstate NY is also in demand -- and this route is about as fast as any other way of getting from NYC to Detroit. Unfortunately there are problems with this (restrictive border policies, deterioration of track in Canada). Suitable funding (which I wouldn't expect from the feds, Michigan, or anywhere in Canada, so it seems unlikely -- perhaps from NY eventually) could buy the Canada Southern tracks and restore the Michigan Central's route.
The Cardinal travels roughly the fourth route which should be operated, though it goes far too slowly and unreliably. I would not expect a lot of through traffic across the mountains, but then, there's traffic from West Virginia into Virginia and DC, and traffic from West Virginia towards Cincinnati, and then there's the corridor from Cincinnati to Indianapolis to Chicago. That would probably exhaust the corridors which can be assembled into NYC-Chicago patterns, although ideally there might be more extensive connections across Ohio and Indiana.
Of course I agree that these routes would be better with some new construction. This can be done incrementally. On the Pennsylvanian, it ought to be redirected from Harrisburg to State College before heading back towards Altoona; this is surprisingly straightfoward and wouldn't lengthen the route, but it would add a whole lot of passengers. Piecemeal tunnels and bypasses could then be used speed up the route from Altoona to Pittsburgh.
