It was a standing-room-only crowd gathered in the aisles of a New Jersey Transit train from New York’s Penn Station to New Jersey at 5 p.m. on Sunday evening.

Multiple groups of friends were overheard asking the same question: Was this because of Taylor Swift?

By now most Americans are well-versed with the Ms. Swift effect. Wherever she goes, whether it’s to perform or even just eat dinner, fans follow. Once word leaked that she might make an appearance at MetLife Stadium for a football game between the Kansas City Chiefs and the New York Jets to cheer on her rumored new boyfriend, Kansas City’s All-Pro tight end Travis Kelce, it was assumed the crowds would show up en masse. The fact that tickets to the game and Mr. Kelce’s jersey — he sports No. 87 for the Chiefs — showed notable increases in sales (and cost), according to multiple media outlets, only confirmed those suspicions.

Ms. Swift did indeed show up at the game and watched it from a private suite with friends Sophie Turner, Blake Lively, Ryan Reynolds, and Hugh Jackman. People in the stadium who were close enough to see the celebrities spent much of the game gawking and taking photos. (Ms. Swift waved to fans multiple times.)

On the train, two Jets fans couldn’t believe their good fortune. “We have four season tickets, and we sold two for $90,” said Michael Callaghan, 30, who lives in Kips Bay. “They were going for $50 before Taylor was going.”

“We should get Swift bracelets that say Jets on them,” quipped his friend, Ryan Dwyer, 36, who lives on the Upper West Side.

At Secaucus Junction, where travelers from Penn Station have to transfer to get a train to the Meadowlands, strangers were joking with one another about whether they were fans of football or Ms. Swift.

When Elaine Gitles, 26, who works for a software company and lives in the East Village, was showing off her red T-shirt that said “Travis Kelce is Hot,” an older man in a green Jets jersey yelled, “It’s a great Taylor Swift jersey.”

Ms. Gitles clarified that she’s a Chiefs fan, and she bought the shirt when her team won the Super Bowl last season. But she’s not mad about being labeled a Swiftie (as Ms. Swift’s fans are nicknamed), since she is one of those as well.

“I went to Eras three times,” Ms. Gitles said of Swift’s wildly successful Eras Tour. “It’s the best coincidence of all time that my favorite musical artist is joining the best football team of all time.” (She also shared the lyrics she came up with that Ms. Swift could sing if her relationship to Mr. Kelce falls through: “He played the field like he played me. A travesty.”)

Other people who stopped to chat in Secaucus were Hana Kim, 24, and Courtney Takahashi, 25, both special education teachers in Westchester. The duo couldn’t afford tickets for Ms. Swift’s concert at the MetLife Stadium in May, so they sat in the parking lot outside the stadium and listened to the whole thing from their cars instead. “We Taylorgated,” said Ms. Takahashi.

“We bought football tickets as soon as the news came out,” said Ms. Takahashi. “I don’t even care if we see her. Just being in her presence, breathing the same air, feeling her vibe, that is good enough for me.”

Outside the stadium, as some fans were tailgating with grills and coolers of beer, others were brainstorming the best way to get a peek at Ms. Swift.

“I’ve been looking at social media, waiting to see if we could see which box she was in and if we could see her,” said Shae Mermis, 33, who lives in Crown Heights and was at the game with her girlfriend and parents.

Stella Fee, 44, and her daughter, Zoe Fee, decided they would hang out on the V.I.P. level. “We are season ticket holders in the level below, so we are allowed to go up a floor to where the boxes are,” she said.

Fans had varying ideas for appropriate game day outfits.

As soon as she saw Ms. Swift in Mr. Kelce’s box last week at the Chiefs game in Kansas City, Caye Schnackel, 26, a fashion designer who lives in Brooklyn, decided she and her boyfriend would have to dress as the celebrity couple for Halloween. “I looked at him and said, ‘I am going to be Taylor, and you are going to be Kelce,’ and that was that,” she said.

She found the perfect outfit at Nova Lee Boutique — a bright red jersey with 87 written on it, made entirely of sequins — and had it overnighted so she could get it in time to wear to Sunday’s game in addition to Halloween.

“I’m just so obsessed with Travis now,” she said. “I am so jealous of Taylor. I am going to try and win him over today.” (Her boyfriend, Krzysztof Miezgiel, who was next to her, just laughed.)

Sean Patrick, 47, went to a T-shirt printing company at a local mall the morning of the game to try and get the saying, “Where’s Taylor?” printed, but he was told it wouldn’t be ready in time. His friend, Tom Wagner, 59, from Manahawkin, N.Y., went to Walmart and bought the largest pink sweatshirt he could find but it still didn’t fit. So they decided to take pink masking tape, write phrases about Taylor Swift on it with a marker, and attach them to football jerseys.

The pair spent four hours tailgating in the parking lot, making pulled pork and chicken on four different grills and watching other football games on a television they set up with a portable satellite dish. “We had about 50 people come up to us and ask to take photos,” said Mr. Wagner. “I’ve never had more fun at a Jets game in my life.”

“The atmosphere is crazy,” he added. “I’ve never seen a team bring a crowd like this.”