The only sensible answer here! Many years ago there was a study by the Central Tranportation Planning Staff ("CTPS") to run the Blue line out to Riverside via the Central subway. (Until fairly recently, the two center tracks at Kenmore were raised in anticpation of high platfrom RTL cars) I recall that the plan was to tunnel under Bowdoin Street. Either way, Red/Blue will be highly disruptive and high cost.