3920 wouldn't be testing with a Type 7/8 though. I'm pretty sure it's just a tracker glitch.
Railroad Forums
Green Line Type 9 Thread
- Discussion relating to commuter rail, light rail, and subway operations of the MBTA.
This morning, 3905-3903 and 3902-3904 trainsets were running at the same time in passenger service. The first time two Type 9 trains made up with active passenger revenue cars running at the same time
3907 is now an active vehicle in the Type 9 fleet which now has 7 active vehicles: 3900-3905, 3907.
It looks like Type 8 3841 has been added to the 3904-3902 trainset according to the tracker of newtrains.today.
According to the inventory page, looks like 3910 is the 10th car delivered this month.
newtrains.today: 3905-3903 all over sudden became 3826-3905 and 3903 as a single car train.
That's a glitch, the Type 9s don't trainline with Type 7s or 8s.
Maybe they were doing a test because they can operate with type 7 and 8 for just coupling but not in passenger service so maybe it was just testing.
folks the 3905-3903 is a pair together. They aren't on any other sets other than themselves. Saw it in park st this morning confirmed!
3902-3904 was running today on the Riverside Line, as far as I know this is the first time the Type 9s have run in service on a Saturday.
For now, the type 9's are running on the riverside branch only between gov't center and riverside which has carhouses along the route both in riverside and reservoir.
Type 7 3684, you are right. On newtrains.today tracker, I saw 3904-3902 running on Saturday afternoon and it was also running this afternoon. Train 3905-3903 ran this morning and went out of service.
This is what I'm starting to notice. Ever since 2902 and 3904 were accepted in July, the type 9s in train service run as two-car trainsets and since 3905 and 3903 have been accepted, they're now running two trains and both are two-car trains on weekday mornings now. So the type 9 service as more cars get accepted keeps enhancing but they don't run full-time service yet because it's the same thing with the new orange line cars how they are watching the performance and improving its performance.
I took the opportunity of a meeting-free morning yesterday and was able to see and ride both of the two car Type 9 sets during the AM rush. Only the third time I'd personally seen Type 9s in service... they're not out at the times I'm usually using the Green Line. They're pretty quiet and seem to have a nice ride quality to them. Those doors are annoyingly slow though, both opening and closing. I suppose it's "safer", but I remember having a very firm respect for the guillotine-chop quickness of the Boeing doors. And even more intimidating were the 01400s with their exposed outside actuator arms. No need for warning beepers or anything like that, just listen for the loud screeee*SLAM* *clunk* 
3905-3903 is on the C-branch. If it's picking passengers, this would be the first revenue service of Type 9's on the C-branch.