A primary source might be:
In all of these cases, primary sources are materials created by participants, observers, witnesses, or recorders. They might be documents or artifacts; they could be textual or visual; and they might be physical (like a letter) or digital (like an email).
You may want to look for libraries, archives, museums, and historical societies in the city or country you are researching. Have any materials been digitized or made available online?
Go broader to get more results. Is (Green Line Boston) not turning up enough results? Does (trolley Boston) or (light rail Boston) work better? How about broadening from (Boston) to (Massachusetts)?
Go narrower to get fewer, more targeted results. Is your search for (transportation Boston) causing you to wade through lots of irrelevant stuff? Make your keywords more specific. If you're actually investigating North Station, try searching ("North Station" AND Boston). You might be surprised at what you get!
Think about words that might address the “who, what, when, where, and why” of your topic. For example:
Who: Massachusetts Bay Transportation Authority OR MBTA (sometimes an acronym will get you what you need, but sometimes you’ll have to search the full name of the agency)
What: the green line extension (broader terms: trolley, subway, rapid transit, public transportation)
When: current, and over the past 10+ years
Where: Somerville, Medford
Why you’re researching this topic (what are you interested in?): gentrification, rising rent, property values, real estate speculation, commercial development
Are there formal and colloquial ways to talk about a specific infrastructure project? For example, what is colloquially called the “Central Artery” is officially known as the “John F. Fitzgerald Expressway.” When doing database or web research, try searching for both.
Who is/was interested in the topic you are investigating? For an infrastructure project, you might think about lawmakers, city or state agencies, community organizers, advocacy groups, neighborhood organizations, and companies that might benefit (or stand to lose).
Where might these people or groups have published writing on this topic? Might they have been quoted somewhere? Have they produced documents that might be collected and made available to the public?
As you learn more about your topic, keep a running list of keywords to try! Because research is iterative, you'll be glad to have a list of keywords to experiment with.