So for many years, all the MBTA commuter rail trains on the Framingham Worcester line had a "P" in front of the number. You can see the "P" in the old schedules. The Framingham Worcester line was the only MBTA commuter rail line with the "P." After the MBTA bought the tracks from Worcester to Framingham, they eventually took over dispatching the line from the MBTA dispatch theatre in South Station. And at that time, since they were no longer governed by CSX rules, they removed the "P" from the train names.
So why do I keep using the "P" in my Twitter posts? I think it helps to clarify that I'm talking about a train number rather than a time. Most people talk about their train according to its departure time - which isn't helpful for the rest of us who may board at a different station at a different time. Since the Framingham Worcester line trains all start with a "5," I think that using the "P" is especially helpful in the afternoon - for example, train P531 departs South Station at 5:35 PM. So some people might get train 531 confused with train 583 which departs South Station at 5:30 PM.
So thanks CSX - I'm keeping your naming convention!
By the way, the South Station dispatch theatre is really cool. My really bad picture of it is here: