Just to clarify...
Tail Tracks or "Dead Ends" exist only at Wonderland, Ashmont, Bowdoin, Oak Grove and North Station (Green). Oak Grove is unique in that it is not used to turn or store trains on a regular basis.
At Ashmont they continue into a yard as they once did at Harvard. Forest Hills, Alewife and Braintree are considered formal "Yards"
Other locations such as Columbia Junction, Quincy Center, Wellington and Orient Heights have "Pocket Tracks" seperate from the main tracks and yards. There were also Dead Ends at Old Forest Hills, Everett and Harvard and Pocket Tracks at Guild Street and Sullivan.
Gerry. STM/BSRA
The next stop is Washington. Change for Forest Hills Trains on the Winter St. Platform, and Everett Trains on the Summer St. Platform. This is an Ashmont train, change for Braintree at Columbia.