The BPL's Norman Leventhal Map Center has created this really nifty thing that you should probably stop reading about right now unless you have some free time, because you're going to want to play with it right away: Atlascope Boston lets you enter a Boston-area address (their maps don't stop at the city line) or location and then see what it looked like in the good old days (some of the maps date to the 1860s).
By default, you'll get a sort of magnifying glass centered on the address or location you choose - which you can move around to see nearby locations.
You can switch to slider controls to see the location change over time - and you can set your own "base" map - you start with a current street map, but can switch to aerial photographs or one of the old atlas maps, which lets you compare how an area looked in two different time periods. For example, in the view below, you can see the South Station area in 1938 (in the circle) and what it looked like in 1888 - note that the location of the tracks is completely different - and that they terminated at a different station:
If you want to learn more, the BPL is holding a session on using the site on Feb. 3 at the Jamaica Plain branch library (it'll be focused on JP locations, but the site's "Search Places" control will let you explore the rest of the Boston area).
H/t Jamaica Plain Facebook group.