When the pressure of a moment in a golf tournament begins to take hold of Aidan Thomas’ mindset, or the muscles that control his swing, he need only recall his early beginnings in the game.

Every now and then, on a big green mat purchased from the nearby Big Five, Aidan, who was 9 years old, and his brother Tyrone, four years older, would each line up five golf balls.

Then they would wait.

When a freight train finally ambled eastward across the Pueblo of Laguna reservation, located 45 miles west of Albuquerque, N.M., the Thomas boys would begin lashing shots from about 140 yards.

“We would hit little low stingers and see how many times we could hit the side of the train before it passed,” said Aidan Thomas, whose family lived on the reservation for a time. “You had to hit them quick. That was pretty fun.”

Thomas needed about five months before he could successfully unload his complete arsenal on those trains.

Now 17, the Bernalillo, N.M., resident has come a long way from those days of hitting off a dog-eared mat and across train tracks. This week, the game has brought him to Baltusrol Golf Club for the 71st U.S. Junior Amateur Championship.

In Monday’s opening round of stroke play — Thomas’ first appearance in a USGA championship — he shot a 6-over 77 on the Upper Course.

Disappointed? Perhaps, but he spoke candidly about the process and the journey. He was not caught up in how this one round affected either.

“I love the challenge of the game,” he said. “I know I make it a little hard on myself, but I am just trying to learn. I am going to play this game until my back is messed up, which hopefully won’t be until I am in my 70s.

“I try not to be a robot. I feel like some kids get into that little mental state, where this is like a job in a way. I don’t like doing that to myself.”

Early on, while living on the reservation, Thomas’ access to the game was limited. There was a hardscrabble municipal course nearby, but it did not offer much in the way of playing opportunities or instruction. So, Thomas relied on his brother, who learned to play from their uncle, for instruction. He would eventually attend a local youth golf program run by the NB3 Foundation, a nonprofit created by Notah Begay III, a four-time PGA Tour winner and past USA Walker Cup competitor, for a couple of summers.

“I should probably still be going,” he said self-deprecatingly when talking about Monday’s opening-round score. “I learned everything from etiquette to how to fix a divot to how to hit shots. I’m not sure I’d be playing at this level without it.”

Along with a menacing stinger, Thomas has no shortage of confidence. When he was in the eighth grade, he watched as Tyrone, then a senior at St. Pius X High, and his teammates gave up a two-stroke lead in the final round of the New Mexico Activities Association’s 5A Championship, ultimately losing by seven strokes.

“I told the coach afterward that I would make sure we would win it the next year [in 2016], if I was part of the team,” said Thomas, now a rising senior. “I knew we were going to be good my freshman year. I was just going to replace my brother’s position.”

Thomas delivered, though his two-round total was 17 strokes higher than his brother’s score from the year before.

Big brother, who played competitively through high school, sees a potential in Aidan that he did not have.

“He’s really good,” Tyrone Thomas said. “He can be a little hard-headed some times, but when he starts listening, he can really put it together. I am impressed, especially qualifying for the U.S. Junior Amateur. That was one of my goals for him because I couldn’t do it.”

Now the question is what’s next?

Stuart Hall is a North Carolina-based freelance writer whose work frequently appears on USGA digital channels.