They are adjusting the schedule to "ease congestion" and to "accommodate the recent Amtrak schedule changes".
The MBTA apologizes for the "late notification".
The changes go into effect on Monday 10/27/08. The changes will be available on the MBTA schedule page on Monday. In the meantime, you should know that:
1 - what was known as the 4:10 PM train (#715) will leave South Station at 3:55 PM (15 minutes early)
2 - what was known as the 4:30 PM train (#799) will leave South Station at 4:30 but terminate at Readville running via the Dorchester Branch.
3 - a new train (#717) will leave South Station at 4:20 PM and make stops at Back Bay, Ruggles, Hyde Park, Readville, Dedham Corp, Norwood Depot, Norwood Central, Windsor Gardens, Walpole, Norfolk, Franklin/Dean and Forge Park/495.