Here’s a great mid-20th Century planning map showing proposed extensions to the Stockholm Metro in 1965. The blue lines show what was already built (or about to be built) at the time, while the different types of green lines show proposed additions at varying levels of planning: solid for planned, dashed for an extension without a full alignment study, and dotted for unstudied alternative extensions. It’s definitely interesting to compare this map to what has been built out in reality!

Source: City of Stockholm Archives