T has to confess: The Blue Line's a mess

The original Blue-Red connector existed as a street-running trolley-pole line with one track that was used for moving Blue Line trains to the Red Line at Charles station so they could be towed to the Red Line repair shops in Cambridge (since relocated). There was never a passenger service between these two. Moves of trains for repair occurred mostly at night. This was because there was no repair facility at Orient Heights at the time. When the Blue got its own repair shop the street running track was closed, ramp to Charles demolished, and the tunnel portal on Cambridge street capped and filled in.

There are good reasons why there is no subway tunnel along that stretch of Cambridge Street.

First, the age fo the buildings along the perimeter are a concern and any disturbance of surrounding soil would likely cause building foundations to crack, tilt, or even collapse. While slurry wall mitigations are a consideration to bolster those foundations the pile drivers to create the wall would themselves cause damaging vibrations that might do as much damage, not to mention a couple of years worth of construction noise above acceptable decibel levels.

Then once you get close to the station you run into the same issues as the Back Bay has; filled land and a high water table. The Boston Ground Water Trust that oversees the ground water needs of the Back Bay would need to run tests to see if ground water is holding up local buildings near Charles station. It is likely that is the case and that may be the deal breaker.

The only workable solution would be a tunnel portal at Bowdoin near where the existing one was, then a gradual incline and surface elevated structure above Cambridge Street. That could then meet the existing Charles station above ground for transfers. Ideally it would be 2 tracks with necessary return switches as found at other MBTA terminus locations so trains can be routed to the correct track. Alternately a single stub-end track may work but at peak service times only every-other train would be able to go there forcing people to change at Bowdoin like they do at Gov't Center to get to Lechmere.

Any Blue-Red connection also needs to address a rebuild of Bowdoin station to lengthen the platforms since the current station does not serve all cars in a train set. A rebuild of Bowdoin has already been depreciated as not happening due to a host of issues with existing foundations, underground utility lines, and more.

While the will of the people may seek this, the ability to actually do this from an engineering perspective, and needless to say cost, may not see this happen. In any event you'd easily be looking at a 4-5 year construction window so local residence and business continue to have access during the process, not to mention input from the fire and ambulance departments and MGH Hospital for access to its emergency department.

While a study may be underway or starting, I fully expect to see the findings as being impossible for these and many additional unforeseen reasons.