On Wednesday, 12-year-old Journey Johnson stepped onto the train platform at Blue Hill Ave. station with a freshly stamped vaccine card in hand and big plans to celebrate the start of an immunized summer vacation: Coffee ice-cream, she said, her “absolute favorite flavor.”

“I thought [the coronavirus vaccine] was going to hurt, but I got it and it wasn’t that bad,” Johnson said.

After a year-and-a-half of remote learning, Johnson looks forward to joining a summer camp and returning to in-person classes at Dorchester’s William W. Henderson Inclusion School this fall.

Johnson was one of the first to receive a dose of the Pfizer vaccine aboard the Commuter Rail as part of the MBTA’s “Vax Express” initiative, a “pop-up” vaccine clinic visiting several locations including Mattapan and the South End from June 16 to June 20. The clinic is sponsored by CIC Health, Cataldo Ambulance, and Keolis Commuter Services, the French-owned company that operates the commuter rail for the MBTA.

Rep. Russell Holmes, whose State House district spans parts of Mattapan, Dorchester, Roslindale, and Jamaica Plain, joined members of Boston’s legislative delegation and transit representatives at the first stop of the “vaccination train.”

Holmes highlighted the urgency of protecting against more transmissible strains and insulating hard-hit communities like Mattapan against further pandemic-induced suffering.

“This station stands in the heart of a community that has been certainly disproportionately impacted by COVID-19 and has certainly still not gotten the vaccinations it needs,” Holmes said.

Ryan Coholan, the MBTA’s chief railroad officer, called the clinic a public victory and said “the opportunity to get the vaccination out to the communities that we serve is so valuable to all of us – ownership and partnership is a two-way street.”

With more than 80 percent of Massachusetts residents having received at least one dose of a coronavirus vaccine and the pace of vaccines seemingly plateauing, the “Vax Express” clinic factors into the state’s final push to immunize stragglers and holdouts.

The clinic dovetails with the state’s new vaccine lottery, “VaxMillions,” in which fully vaccinated residents can try their luck at a million-dollar prize, beginning July 1. Those younger than 18 years can throw their name in the hat for a chance at winning a $300,000 scholarship, but Gov. Charlie Baker’s reminder goes for all ages: “If you’re not vaccinated, you can’t play.”

Rep. Holmes left his own reminder for legislators eager to pat themselves on the back for high but disparate vaccination rates – “We may be number one in the country in some respects, but that falls on deaf ears if it’s not being felt in the community,” he said.

The “Vax Express” will chug along for a second round of vaccine doses on July 7 and July 11 at Blue Hill Ave.