More specifically, D1 refers to "District 1", which is a term originated at Guilford Transportation Industries in the 1980s to refer to the dispatching district then comprising the former Maine Central Railroad territory. So, here, if you see reference to D1 or District 1, the poster is usually referring to the historical GTI/PAR dispatching territory timetable east of Portland, Maine. Broadly, District 2 was/is the former B&M territory between Portland and Ayer, and District 3 was/is the old "West End" or Fitchburg Division west of Ayer and including the lines in Connecticut. There are changes and exceptions now that Pan Am has been sold to CSX and the former Pan Am Southern has gone to G&W/B&E operation (for example, CSX assigned the ex-MEC Brunswick Branch to District 2 and the former B&M Worcester Subdivision to District 1) but generally, if you think "Maine Central" when you see "District 1" or "D1" here, you're likely in the right place.