FWIW, I saw the alerts this morning for 505 and given dbperry's write-up about turns of equipment figured the 516 would be late. The T alerts verified this fact, so I figured I'd wait at home and catch 516 at Westborough in 45 or so minutes. Turns out it was 1 hour and 3 minutes into the station with a consist of 2 trains according to conductor's announcement apologizing for the delay. Doubles on both ends plus 2 engines(!) but I haven't verified as I got on "in the front". Anyway, as we leave Wellesley Square, we're 1 hour 1 minute late. Nice to have a double to sit in as this usually all singles.
After getting off at Yawkey, I mistook the "doubles on the ends" as the whole train without seeing that this was just P507 and P505 joined together, which I got from reading dbperry's blog about P508's delay because of P505 breaking down. We were riding in the P507 part of P516. And it was not an unpleasant trip at all, unless you had to wait the hour at the station.
Will they de-couple these once they get to South Station? Or does it all depend upon available equipment or other factors?