One problem with running some shuttle buses from Fenway & St. Mary's all the way to Boston Common is fare collection. There would need to be an open fare gate at Park St. for shuttle riders continuing a subway ride.
In the instance of reconstructing the Beacon Junction the fare collection complexities were limited to Kenmore Station. (Typically during bustitution no one collects fares at Fenway like they do after Sox games.)
When a transit system settles into a weekly schedule, it can become a somewhat sustaining customer serving entity with orderly fare collection. When we add large events and create the need for a citywide ad-hoc take-all-comers transportation system we need to change the role of the system from a somewhat sustaining entity to a public service. As a public service it can expect to have added infusions of tax money to make up for making fare collection secondary to optimizing the other aspects of service.
I read an article where someone said that Uber was adding greatly to traffic congestion. Much of that congestion happens due to fare collection, where the vehicle sits for awhile to accomplish this. How many times have you ridden a streetcar and waited while someone needed help from the motorman to pay his fare? Incidently fare collection slows down all modes of transportation -- commuter rail, buses, streetcar/light rail, traffic signal pre-emption, regular taxis, and Ubers.
Getting back to the commuter rail, it has been proven in Boston that finding a suitable time and place to collect fares (geographical place on the landscape describable in metes and bounds accurate to 10 feet or describable using latitude and longitude accurate to 4 decimal places) is hard enough with normal rush hour rider loads let alone with event crowds descending on the station. Witness the suspension of fare collection on the evenings of Dec. 31 and July 4.
Quick! Stop your bank account autopayments so you can decide where you want your unemployment check or stimulus payment to go first. (You might choose to set credit cards to minimum payment).