It was the perfect synthesis of song and performer in that Motown studio one sultry Detroit day in 1967. The song was the work of one of the label’s steadfast songwriting duos, Barrett Strong and Norman Whitfield, called “I Heard It Through the Grapevine.” A smoky, gospel-tinged song reminiscent of mid-’50s R&B, it was by now on at least its third contender among Motown artists. First had come The Miracles, who tried the song at a fast, rollicking clip typical of the group’s style. It didn’t work, so up next came Marvin Gaye: he slowed it down, playing the track as a moody, almost jazzy number. The results were compelling—but they weren’t, to the ears of Motown’s notoriously stringent chief, Berry Gordy, the kind of stuff hits are made of.

But Whitfield was sure he had something and could find an artist who could be the song’s match. Enter Gladys Knight, Bubba Knight, William Guest and Edward Patten—collectively known as Gladys Knight & the Pips—who convened at Motown’s recording complex in mid-June to set down their intricate vocal arrangement for the song.

For weeks, the group ran drills on their complex vocal arrangements. The result was two minutes and 47 seconds of percussive, propulsive soul: faster and more animated than Gaye’s arrangement, but with the same minor-key tension and unresolved urgency that hooks the listener from the opening verse. Knight’s lead vocals, the mounting volume of the backing Pips, the piano and horn section building minute by minute into a wall of sound—it all came together in one perfect package. In her autobiography Between Each Line of Pain and Glory, Knight remembered the moment it happened: “Norman was kicked back listening and trying not to smile But we could tell we had him. When the guys got to ‘Just about, just about, just about to lose my mind,’ we knew Norman had made up his. We had a hit and he knew it. He could hold it no longer. He shot out of his chair: ‘We’ve got to get a studio right now.’”

“I Heard It Through the Grapevine” would hit the #1 spot on the R&B charts and remain there for six weeks running. By the end of the year, it was Motown’s top-selling single, well on its way to becoming a staple of soul artists on and off the label. Motown, soul listeners, and the world had discovered something new. Gladys Knight, the artist who felt Gordy had placed her at the bottom of his recording totem pole, was a name his listeners weren’t about to forget.

What makes that moment such a pivotal point in the history of soul music was the inflection it represents in American musical culture. Knight would become one of the key figures in making soul music not just another flavor of mainstream pop, but something definitively and undeniably born out of Black American passion, culture, pain and joy. And both Gordy and the whole music industry would follow that lead.

Knight’s childhood had been spent in Atlanta, her family a part of the city’s growing class of upwardly mobile middle-class Black residents. One generation removed from rural poverty, her father had become one of the city’s first Black post office employees: a prestigious government job at a time when Jim Crow governments were working hard to keep such opportunities off-limits.

Perhaps this was why Knight and her siblings found themselves, not always willingly, drilled, practiced and displayed as future musical virtuosos from an early age. Like many young singers of her background, Knight gave her earliest performances at the pulpit of her church, singing the gospel melodies that would inform her artistic ethos decades later. But by age seven, she—along with her siblings and two cousins—were performing semi-professionally under the name the Pips: courtesy of a nickname of cousin James.

A few hits came early on minor labels like Huntom, Fury and Fire, but for artists precariously poised between Southern poverty and national fame, it wasn’t the break that was required. “We were getting a lot of airplay,” Knight recalled sardonically in her book. “But we couldn’t buy food with airplay.” To get over that wall, the Pips were going to need to reach buyers well beyond the Black core of R&B audiences, and into the white listening mainstream of the Hot 100. And for soul artists in the mid-1960s, that meant one label above all others: Motown.

From the label’s beginning, it was known for its distinctive sound—the Motown sound—which was a calculated invention. Rooted in the same gospel call-and-response structures and simple chord progressions as earlier R&B, what had formerly been organic and sometimes accidental became (under Gordy, Smokey Robinson, and the wildly prolific songwriting team of Eddie Holland, Lamont Dozier and Brian Holland) a finely tuned science.

There is no sense in questioning whether it worked. In 1964, Motown produced five Top 10 singles; a year later, they doubled that count. By 1971, Motown had put out no fewer than 110 songs that entered the Top 10. But the process had its detractors, even at the time. Motown was as much an assembly line as the nearby General Motors and Ford plants. Artists recording the hits could sometimes feel like interchangeable song-and-dance machines, an impression that Gordy’s comments didn’t always help assuage: Gordy would tell one reporter in 1965, “Some of these artists would be waiting on tables somewhere if there hadn’t been a place in Detroit to recognize their talents.”

Just as importantly and controversially, Motown’s popularity depended on reaching the ears and wallets of the vast bulk of white record buyers, and the label unquestionably knew it. The music they played was Black, no doubt, but it was a sort of pop-inflected version of R&B not audibly unlike that which had been produced by white songwriters like Goffin & King early in the decade.

This was the Motown into which Gladys Knight & the Pips were coming in 1967. Except with one big difference: The America of 1967 wasn’t the America of Motown’s early years. The same summer that the Pips entered the studio to record “Grapevine,” a national spate of more than 150 race riots—predominantly northern, urban and fueled by the abuse and injustices visited on Black ghettos far from the old protest epicenter of the South—had been dubbed the “long, hot summer” by the press.

It would have been impossible for Motown to survive if the music hadn’t mirrored this change. In 1966 and 1967, Gordy’s label was facing growing, stiff competition from more rootsy, gritty and unapologetically Southern varieties of soul, thanks to the output of artists like Otis Redding and Sam & Dave on Stax or Aretha Franklin on Atlantic.

The Pips’ earliest pop-inflected attempts to craft a viable single have all the makings of an early-’60s Gordy production. There’s nothing wrong, per se, with “Just Walk in My Shoes,” the group’s first single after signing to the northern label. But there isn’t anything distinctive about it, either: it’s all gloss and sheen, with a drumbeat meant to remind us of The Supremes and backing vocals meant to remind us of The Vandellas. It feels just a little too precise, a little too deliberate, a little too Detroit.

So in one sense, “I Heard It Through the Grapevine” was a turning point not just for Gladys Knight & the Pips as artists, but for Motown as a musically relevant concern. Knight knew it, too. “We had a great feeling about it as soon as we heard it, and we felt that once we added some background vocals, it would be even better,” she recalled in her book. “We treated that demo tape as if it was the Hope Diamond. It certainly was the hope for the future of our career.”

The theory proved correct: “Grapevine” would go on to be the highest-selling R&B single of the year and the first of what would come to be identified as a second phase of the Motown sound. Ironically, Motown’s gain would be Knight’s loss: While the song had been a great enough hit to make the Pips a household name, it would be an even bigger hit the following year for Marvin Gaye. And this, indeed, was the name of the game for Berry’s label. To a great extent, the sound that Knight had produced with this track would become the template for the Motown of the later part of the 1960s and the early 1970s: rougher, bluesier, looser and more willing to accept and nurture the distinctive, personal tone and quality of its individual singers.

In 1973, dissatisfied with Motown’s failure to offer them the greater promotion (and bigger contract) to which they felt their new fame entitled them, Knight and her group decamped for Buddah Records, and it was here that the kind of soul Knight embodied would reach its full apotheosis with “Midnight Train to Georgia” With deference to Ray Charles, “Midnight Train to Georgia” remains a strong contender to this day for the title of iconic song for the Peach State.

It’s interesting to note that the song wasn’t meant to be about Georgia at all. Originally written by Jim Weatherly under the title “Midnight Plane to Houston,” it was changed to its more familiar refrain by Cissy Houston, who first recorded the track. It’s hard to overstate the outsized importance of that one small change. One person who thinks so is producer and engineer Ed Stasium: “If it was still titled ‘Midnight Plane to Houston,’ it would not have been a hit,” Stasium told HITS. “It’s so romantic: you know, a midnight train. When I was a kid, there were still steam trains on the tracks in New Jersey. It’s this romantic idealism, traveling on a train—it’s mystical, almost.”

Today, Stasium is best known as one of the formative sonic architects of the punk and new wave scenes, having worked on albums by the likes of the Ramones, Talking Heads and The Misfits. But in 1973, he was just starting out as a musician and engineer when he got the call from producer Tony Camillo to come into the studio for a set of sessions with Gladys Knight & the Pips. As Stasium recalls, crafting the song successfully was a matter of trial and error.

“We tried two different versions that Knight did not dig,” he told us. “They were slower versions—more on the level of a ballad. And Knight wasn’t happy with that: She wanted more of an Al Green vibe.”

Therein lay the key to the unforgettable opening hook of “Midnight Train”: Take those opening horn bars of Al Green’s “Let’s Stay Together,” speed them up in a major key with a piano underneath, and you have Camillo’s iconic opening arrangement. Knight was sold.

“The following Monday or Tuesday, [musicians] all came down from New York,” Stasium told us. “We put the horn section in, put the strings on. Barry Miles, jazz keyboard player—he dubbed the acoustic piano. And Camillo did a little bit of percussion and put a little bit of B-3 [Hammond organ] in there. And he sent the rough mix off to Knight and she loved it.

“A couple of weeks later, on my first plane ride ever, I flew out to Detroit to a studio called Artie Fields and did the Pips’ backing vocals. We punched in practically every line, and then double-tracked it. You know: ‘Leaving on a midnight train to Georgia’—stop. Get it right. Double-track it. Go on to the next one. And then Knight says, ‘Okay, I want to sing now.’ And she did the vocal in one take. One take! We punched in one line, because she didn’t like one of the vamps at the end.”

“Midnight Train” was the first song in a long time that felt like it truly belonged to us,” Knight recalled in her book. “Most of all, it broke through all barriers and broke through on to the pop charts, proving once and for all we were nobody’s stepchild.”

This is the ineffable magic of the success of “Midnight Train,” which would be the success of 1970s soul music in microcosm: In its unmistakable, personal specificity, it becomes universal. It is the most passionately Georgian song that could be designed, but in its rootedness within Knight’s own experience and cultural milieu, it becomes something that belongs to the world. This was a new kind of R&B, one that didn’t court white record buyers by pandering to them, but rather by opening a window into a music and background they would otherwise never have been able to access. It was Knight’s legacy to pop music.

Knight’s risky divorce from Motown proved to be a wise career move; she and the Pips had nine more top-20 singles before the ensemble dissolved in 1989. She has, since then, settled into the somewhat familiar pattern of soul music elder stateswomen: inductions into the Rock & Roll and Soul Music halls of fame, a star on the Hollywood Walk of Fame, a Kennedy Center Honor and more than one final farewell tour with her group. It’s a fine denouement and one that any artist would be proud of. And every time you hear the opening percussion and assured vocals of “Grapevine,” it’s a reminder of the moment in Gladys Knight’s career that mattered most: the time when she and her group changed the direction of Motown, of soul records and of popular music.