Update: I haven’t worked on this in many moons and it has ceased to work. Here is a currently working MBTA map.
The coverage area has increased to cover Boston, Charlestown, South Boston, and Brookline (the 4 downtown maps). Rather than creating the slices for tiles by hand in Photoshop, I whipped up a PHP script to do it for me - slicer.php (requires GD w/ GIF support, an increased memory_limit in php.ini, and a GIF version of full.psd (same dimensions)).
I added zoom level 1 by importing the PDFs as 3500px wide images and providing alternate coordinates to the slicer. Hint: it’s much faster to work with them after converting them to Indexed Color (via RGB, as they get imported as CMYK) (and obviates the need to Export to Web).