WHEN MBTA riders begin Monday to hop onto Green Line trolleys at a pair of brand-new stations, they will also experience another first: a different way of tapping to pay their fare.

Green Line extension passengers boarding at Somerville’s Union Square Station or the newly-constructed Lechmere Station in Cambridge will need to head to their stop’s fare vending machine and acquire a paper receipt proving they paid, a departure from the typical fare gate present at most stations or the on-board fare boxes used at other above-ground Green Line stops.

The MBTA tweeted out a video on Friday afternoon, less than three days before service to the new stations is set to begin early Monday morning, with instructions for passengers.

Commuters boarding at Lechmere and Union Square should tap their CharlieCard or tappable CharlieCard at a fare vending machine, select “validation,” tap again, then take a printed receipt demonstrating proof of payment onto the trolley.

A T spokesperson said the validate-and-receipt system will serve as an “interim process” for collecting fares at the two new stations. The stops were designed with a future fare infrastructure in mind that will allow riders to tap their cards at any trolley door and board, but the Green Line Extension is running “ahead of the phased roll-out” of that fare overhaul.

All riders at the two stations will board from a center platform, which means they will not pass directly by a farebox as they do at other stations. If they choose to do so, they can still walk to the front of the train to tap.

Asked how the T would handle riders who skip the validation process, MBTA spokesperson Lisa Battiston replied, “We encourage our riders to validate at the fare vending machine prior to boarding or at the farebox at the front of the train. Riders who have not validated at a fare vending machine are asked to pay their fare at the farebox on board the train.”

The first branch of the Green Line Extension will open to riders at the start of service Monday, featuring the Union Square and Lechmere stops as well as reopening of Science Park. Starting that day, Green Line trains on the B, C and E Branch will end at Lechmere, while D Branch trains will continue onward to Union Square.