I'm trying to assume a complete history of Park Street from various sources on Wikipedia;
here's what I've got so far. I have a pretty good understanding of how the station was modified up to about 1980, but I can't assemble a good timeline past then.
I have the following as current sources:
*From the NETransit history, the Red Line platforms were extended northwest sometime between 1984 and 1987 to accommodate 6-car trains.
* I have 1989 and 1990 maps in various formats (and most other maps after) explicitly designating it as handicapped accessible, and a 2000 map claiming accessibility for the Red Line level only.
*As of
this 2007 report, elevator #804 (street to Red Line fare lobby) dated to 1991, #812 (GL eastbound to tunnel) and #823 (GL westbound to tunnel) dated to 1993, and #808 (fare lobby to RL) dated to 1987.
* Elevators #978 (surface to GL westbound fare area) and 979 (GL westbound to RL) opened on December 21, 2012, per MBTA press release.
So, it sounds like there were three separate renovations: the original four elevators and the RL platform extension in the 1980s, more modifications around 2003 (which i can't find any sources for), and the two additional elevators in 2012. Does anyone have further details / sources?