AUSTIN -- Mumford & Sons, Edward Sharpe and members of Old Crow Medicine Show had themselves a Railroad Revival reunion during SXSW, and like many family reunions, they took the opportunity to introduce new additions. The roots-based groups convened on the lawn of the LBJ Library, where their road doc "Big Easy Express" filmed in advance of short sets from the former two full bands, a presentation by MySpace.

Mumfords performed three new songs from their forthcoming sophomore set. "Lovers Eyes" had a little of the "quiet rage" that its lyrics suggested, frontman Marcus Mumford lamenting "Love was kind for a time… we're too young / our heads too strong / to bear the weight of these lover's eyes." It, like a handful of songs from "Sigh No More," gives way to the stomps and the sixths, all four singers busting out into harmony. "Lover of the Light" featured Marcus behind the drums for an overall soft-rocker with a big build, and "Ghosts That We Knew" falls in line with some religious-tinged imagery in the British band's songs, about hard hearts, soft lovers, darkness and light:. "You saw my pain washed out in the rain… saw blood run from my veins."

Tipping their hat to one of the most memorable moments in the film, the band then re-introduced the Austin High School marching band to help out on "The Cave."

Edward and his Zeros also bowed a new song, apparently and appropriately titled "Best Friends Forever," though with lesser practice and effect than the Mumfords. Alex Ebert and Jade Castrinos shared leads, duetting to an easy country rhythm, though the vocalists struggled with lyrics and the band had difficulty shining it off.

"We have a bunch of new shit," said Ebert, "but we don't know it very well."

Their whole set seemed to be fraught with a little bumbling, even as a delightfully well-rehearsed and entertaining as the band usually is. The just didn't know when to finish each song, like on "Up From Below" and even fan-favorite "Home."

However, the volatility only made for a jubilant encore, with Mumford & Sons inviting up Sharpe and Gill Landry of Old Crow for "Wagon Wheel," the latter band's immensely popular southern-loving hit.

Comparitively, "Big Easy Express" was just an overview, an appetizer. It followed all three bands' short, southwesterly tour journey, traversed on a California Zephyr train. As I'd previously surmised, it was like three peas in a pod, the growing relationships between the bands bordering on twee levels of giddy. Shot in a variety of color contrasts and flipping occasionally to black and white, it came off more as a series of Instagram pictures with a very sincere, love-positive soundtrack than a traditional rock doc or concert film. It was: band playing in a round in a field; band performing earnest songs about trains; band playing biggest hit; musty vintage polyester and big hats, from the mountain to the prairies to the ocean.

But each band's charm is obvious, its frontmen relating the showman's version of their romantic trip. It tapped into a nostalgia for the Lost West, a dotted line between throwback-cool and itinerant-chic from city to city.

With new records on the way from Mumfords and Edward Sharpe, and Old Crow Medicine Show eternally on the road, they'll have their chances to perhaps reconvene (again) to relive some of that magic. The concert performances may have been a little awkward, but then again, most family reunions are.