There's no doubt that the B&A lost capacity when it was single-tracked. But, traffic density has been well within the capabilities of post-WWII traffic control systems to reasonably handle the requirements on a single-tracked railroad with well-spaced, long sections of 40/50 mph second track. As a matter of fact, the NYC wanted to single track the B&A back in the 1950's, but capital constraints and economic payback versus other projects didn't allow it until finally Conrail took the project on once they were starting to make good money and the signal system needed replacement. I don't recall that B&A speeds were raised as a result of this project, but the computer simulation-generated configuration of single/double track, CTC, bi-directional signalling, and 45-mph turnouts helped to keep things moving to offset some degree of the increase in train dwell time resulting from the single-tracking, and especially with the flexibility it afforded during times of maintenance or for allowing faster trains to pass slower trains. In any case, as has been mentioned, whether/to what degree CSX would agree to increased passenger traffic on the B&A and whether they would require additional double track (triple between CP-60 and CP-64) has much to do with the direction and time slot of the proposed passenger trains, especially how it would mesh with its intermodal traffic.
Since Amtrak's creation, there have been 2 eras of inland route service, both funded by the Commonwealth of MA, if I remember correctly. The first was a roundtrip service in the mid-70's that was discontinued when state support ended. The second, with 2 roundtrips/day, ended in the early 2000's, as I understand it, at CSX's behest because of capacity and/or interference issues. While both inland route services included a westbound morning schedule running against eastbound intermodal traffic, especially with the high-volume, tightly-scheduled UPS traffic out of Chicago/gateway into New England with service commitments, since single-tracking this provided interference. In the same vain, passenger service scheduled to arrive eastbound into Boston during the evening ran against westbound intermodal service.
In terms of the capabilities of the B&A vs. the B&M, modern operating and contemporary traffic growth capabilities have tilted the scales in favor of the B&A. Modern locomotive technology has/will continue to narrow the gap in issues of grades and profiles, and the necessary characteristic to support the boom in rail traffic forecast during the forseeable future, the ability to clear overhead clearances to 20'6"+, gives the B&A an edge. While the B&A is already being cleared to 20'6"+ (for ~ $50m.), capable of handling domestic containers doublestacked, the B&M is hamstrung by the economic (possibly technical) barrier of clearing the 1880's era, 5-mile Hoosac Tunnel (which alone could be 2 or 3 times the cost of doing the entire B&A to Worcester) to accomodate the doublestack configuration that allows for this growing traffic converted from over-the-road trucking with high economic returns.
How additional passenger service would mesh with CSX's important New England intermodal franchise, with its existing high-revenue, high-volume premium traffic, and for which they're spending/receiving negotiated benefits of a ~ $150 million investment in the B&A to double intermodal volume capacity to handle new traffic , IMHO, would inform their position with any proposed increase in passenger trains west of Worcester.