parovoz wrote:So, why not use the Bowdoin tracks for storage, then? I assume the existing storage area would remain accessible to the motormen?
The existing tail tracks lie in the footprint of the extension tunnel and therefore would be likely be severed. Depending on the relative depth, method of construction, and whether or not a station is retained at Bowdoin, the tunnel structure surrounding the tail tracks may also be destroyed or rendered unusable by the extension construction.
It's also unclear to me whether the Bowdoin loop and tail tracks would even remain usable during construction, or if trains would be have to be turned at Government Center. With only a single crossover at GC, only the Bowdoin-bound track would be usable for short-turns - not sure what effect this would have on throughput.
Another note on the tail tracks - when unoccupied it's possible that they would allow terminating trains to enter the station at higher speed as a signal block could exist between the end of the platform and the end of track. If the tracks stub out within the station, trains would likely have to enter at a restricted speed (e.g. prepared to stop, as they do at North/South station).