P500 & P502, the first two inbound trains, departed Worcester on time. Both those trains use equipment that originates ("lays over") in Worcester. Once P500 reached somewhere near Wellesley Square, it was blocked by a tree across both tracks. P502 eventually ended up directly behind P500.
This tree must have fallen down between about ~5:15 AM and 5:45 AM. This is because there were two outbound trains that passed through Wellesley without any problems before P500 found the tree. P501 departs Boston at 4:30 AM and arrives at Worcester at 5:35 AM. P581, a 'non-revenue' / positioning / deadhead move then departs Boston at 4:45 AM and arrives in Worcester at 5:53 AM. So I'm guessing that P581 went through Wellesley sometime around 5:00 to 5:15 AM.
P501 turns at Worcester to become P504, while the non-revenue P581 usually turns at Worcester to become the P582 express.
At just about the same time that P500 found its path blocked by the downed tree, outbound P501 died somewhere during its outbound trip. Reports indicate the locomotive had a problem with the main reservoir - a component of the compressed air braking system. P581 pulled up behind P501, coupled to it, and shoved it to Worcester.
Perhaps they thought they could fix the problem and keep all the trains operational, but that wasn't possible and they eventually cancelled P504. They also decided to convert P582 to a local to gather up all the stranded and waiting P504 passengers. In either a stroke of luck or good planning, they decided to use the usual P506 equipment (six doubles) which was already at the Worcester layover yard as the delayed and now local P582. P581 / P582 is planned to have 4 doubles and 2 flats, but I'm not sure it has had this consist lately. The P581 equipment was used as P506.
It took about 40 minutes to clear the tree from the tracks in Wellesley. P500 was on the move around 6:30 AM with P502 directly behind it.
It is interesting to note that P504 and P582 are scheduled to pass through Wellesley around 6:45 AM, so they probably would have been caught at the downed tree / P500 / P502 traffic jam had they operated on time. But P504 and P582 probably only would have been delayed about ~10-15 minutes in this scenario. The P504 passengers were actually delayed about 40 minutes after being forced to take the delayed P582. So P504 and P582 passengers were definitely delayed more than if the downed tree was the only problem.
The good news was that all the problems were resolved and cleared out by the time P506 and P508 came through and neither of those trains were delayed.
Alert rider Dmitry Lev noticed a "lonely locomotive trudging west near Yawkey around 10am." I wonder if that locomotive was going to Worcester to retrieve the P501 / P504 equipment to bring it back to Boston for repairs.
P500 passengers: 40 minutes late to Boston
P502 passengers: 15 minutes late to Boston
passengers intending to take P504: 30 minutes late to Boston
P582 passengers: 40 minutes late to Boston
Just for reference, according to the June 2015 manual passenger counts, each of those trains has the following typical passenger load:
Total passengers delayed: ~2000
These counts are generated by the Keolis customer service agents with the manual 'clickers' counting passengers as they disembark in Boston. Probably the most accurate count that is available, and the most recent data available.