It appears that you can now take MBTA commuter trains free of charge between Back Bay and South Station, and from the wording at MBTA's website, it seems to be free in that direction only.

This is an effort to promote tourist use of the train between Back Bay hotels and South Station. MBTA's website is silent as to whether customers should just board, or obtain some type of free ticket prior to boarding.

It will be interesting how tourists are supposed to know which platform at Back Bay Station they are to board from, as South Station bound trains can leave from one of two platforms. In the past this information was not posted on the TV monitors at BB station.

http://www.mbta.com/insidethet/press...ls.asp?ID=1269