Chambers Bay’s pecu- CHOOO CHOOO! -liarity is quite a treat

The U.S. Amateur field was down to a pair of twosomes Saturday, yet it was still possible for the 5,050 fans on hand to savor the quirks that define Chambers Bay from, oh, just about any golf course on the planet.

Take the 17th hole, a 218-yard par-3 nestled against the shoreline railway tracks. (Hence the hole’s nickname, “Derailed.”)

As David Chung was about to pull his club back for the putt that would’ve clinched his semifinal match against Byeong-Hun An, a BNSF locomotive chugged by the gallery, announcing its presence with a couple of horn blasts loud enough to stir Old Tom Morris out of his grave at St. Andrews Cathedral.

Golfers don’t like distractions, which is why it is preferable to trip into a mound of fire ants than to suffer the humiliation of owning a cell phone that blares midway through a player’s backswing.

Although the train whistle that startled Chung qualified as a distraction, the affable Stanford star could only smile about it afterward.

“He gave me a few honks,” Chung said of the engineer, whose day was probably made by the sight of a golfer flinching in preparation for a putt. “It was a long train, too. It didn’t stop for a while. So I just decided to hit it.”

Chung’s opponent today in the 36-hole final, Peter Uihlein, was similarly serenaded by a locomotive on the same hole Friday. He tuned it out.

“But I noticed the helicopters on 11,” Uihlien said. “Those were pretty cool.”

Choo choos and choppers are just part of the industrial-strength charm of Chambers Bay, home of giant concrete sorting bins that acknowledge the site’s mining-operation heritage, a walking path that snakes through the links and is open to the public (though not for this event) and a lone fir tree, perhaps the course’s most identifiable landmark.

Golf purists loathe gimmickry, but the beauty of Chambers Bay is that its signature nuances really aren’t gimmicks. Those locomotives that break up the intense silence before a clutch putt on No. 17? They’re a blast from golf’s past, an homage to such distinguished havens as Prestwick, Scotland, and Australia’s Royal Adelaide.

And while Chambers Bay was prepared to induce frustration during the medal rounds of the U.S. Amateur, the golfers who advanced to match play on Wednesday found a defanged layout that rewarded those who took risks. It was as if the USGA had made its point – the fescue greens can be tweaked to make the course as difficult as needed for the 2015 U.S. Open – and then gave the tournament back to the college kids.

But remember: The course that An dominated early Saturday morning – the defending champion was 6-under after six holes – isn’t necessarily the same course that Chung and Uihlein will encounter in their showdown for the Havemeyer Trophy.

“By this time in the tournament, when you’ve played four, five tournament rounds, you’ve studied the greens and the course enough to pretty much (know) every part of it and to know what to expect,” said Chung.

“But every time I come to this course, I’m surprised by something new about the greens or about the fairways. There’s something new to learn about this course every single day.”

The No. 5 hole, for instance, was built with an alternate green put to use on Saturday, reducing its distance by nearly 200 yards.

No. 18 is sometimes a par-5 and sometimes a par-4. During the semifinals, it was a par-4, adjusted by moving up the tees to bring the right bunker into play while allowing golfers to carry the left bunker and catch “the speed slot” on the fairway.

“I was kind of hitting myself on the head for making a stupid error on the approach shot,” said Chung, who still ended up winning the finishing hole and advancing with a 1-up decision over An. “But I never played from that tee, and I didn’t really know.”

Chung, who already this summer has won the Western Amateur and Porter Cup – in addition to helping lead the United States to a victory over Europe in the Palmer Cup – won’t be blindsided by any radical set-up changes in the finale. Nor will Uihlein, an Oklahoma State junior whose resume includes the 2010 Sahalee Players Championship.

“The tournament began with 312 players and a lot of them had never played links golf before,” said Brent Zepp, the pro at Chambers Bay.

“They didn’t understand the links game – they were fighting it, relying on flop shots,” Zepp continued, referring to those vertical approaches designed to arrive on the green with minimal movement.

Flop shots can be a sensible option on a conventional course, but at a links-style layout such as Chambers Bay, where the long greens are rife with ridges and subtle contours, the flop shot often is a prelude to trouble.

“The guys who stuck around know how to hit knockdowns and roll the ball,” Zepp said. “That’s why the average score went from 80 earlier in the week to what we saw today, with An and Chung playing around 5- and 6-under.”

On Friday, after qualifying for the semifinals, An described Chambers Bay as “really fun … more fun than hard, I think.”

The forecast today is calling for a bit of both: Some fun will be had between Chung and Uihlein – longtime friends whose advancement to the finals virtually guarantees them an invitation to compete in the Masters next April – but the degree of difficulty figures to be amped up, as well.

And if a train whistle interrupts somebody’s focus on a putt at No. 17, it’ll be another reason to appreciate the grand funk of a golf course built next to a railroad track.