Submitted by emotingviamemes, who says:

This is an old Boston Elevated Railway Company map from a user guide I purchased at the wonderful Ward Maps store in Cambridge, MA. It’s a lovely relic of its time! The rest of the guide gives transit directions to landmarks and points of interest. I’m not exactly sure why the modern day Blue and Red Lines are the same color on here, unless some color had faded with age.

Transit Maps says:

Yep, it’s a beauty alright!

I’ve given up trying to comprehensively date Boston maps (one of my readers always comes up with more accurate dating than me!), so I’m just putting it in the rough range of 1928 to 1938, mainly based on the existence of the Atlantic Avenue Elevated line.

Note: Steven Beaucher from Ward Maps has identified this as the 1932 map for me (see, I told you someone else would know!).

Regarding the route colours as shown, I’d just say that it was done in an effort to minimise the number of colours used in the print job. The “yellow” and “red” lines on this map run concurrently with each other between Haymarket and North Station and thus need some visual differentiation to make them easy to follow, while the two “blue” routes only cross other routes and don’t interact with each other, so they can safely use the same colour. Assignation of route colours back in those early days of transit map design was quite random: even early pre-Beck London Underground maps could never really decide which line got which colour. And remember that Boston’s modern route colours were only defined in the late 1960s when the famous “spider map” was introduced.