Next stop: Somerville. The Green Line Extension, which has been in the works for three full decades, finally opened for service on Monday. Here’s what to know before you take your first historic train ride.

🚆In total, the project is worth a whopping $2.3 billion, and expects to support an increased ridership of 50,000+ trips per day.

🚆The brand new station at Union Square is now open in Somerville, as well as the relocated Lechmere Station in Cambridge. Regular service between North Station and Lechmere has also resumed.

🚆To get to Union Square, riders will need to take the E Branch trains, as D trains end service at North Station, and the B and C trains stop at Government Center. Learn more about the branch destinations here.

🚆With brand new stations, comes a brand new way of paying. While trips still cost the same amount, riders will now have to validate CharlieCards and CharlieTickets at a fare vending machine prior to boarding at Lechmere and Union Square. The MBTA has whipped up a quick video tutorial so passengers can stay on the right track.

🚆Later this year, there will be five more stations with service on the Green Line: Somerville, Gilman Square, Magoun Square, Ball Square + College Ave. in Medford, right by Tufts University. The remaining stations are expected to open this summer.