For all its frictionless rhythms and gleaming surfaces, Kraftwerk’s catalog can be a maze-like affair. The Düsseldorf group’s first three albums (1970’s Kraftwerk, 1972’s Kraftwerk 2, and 1973’s Ralf & Florian) have long been unavailable, perhaps because of the gulf between the LPs’ avant-garde rock and the sleek electronic pop for which Kraftwerk would thereafter become famous. As they grew, the group modeled their image and production alike upon the efficiency of business and industry, and like any successful corporation, Kraftwerk were never shy about reinventing themselves. If their electronic period began with the pinging arpeggios of 1974’s Autobahn, their synth-pop era kicked off in earnest with 1975’s Radio-Activity, where they explored shorter songs and sharper hooks. But with 1977’s Trans Europe Express, they perfected their fusion of electronic experimentation and futurist philosophy. Trans Europe Express situated Kraftwerk as something like Germany’s answer to Andy Warhol: In music for the masses, they invented a new kind of conceptual art. The album includes not only some of their most consequential sounds—the title track’s melody would form the basis for Afrika Bambaataa’s “Planet Rock,” while the grinding rhythm of “Metal on Metal” would help shape the birth of techno a few years later—but also the ideas that would distinguish Kraftwerk as pop music’s resident intellectuals. As on Autobahn and Radio-Activity, the group remains committed to the fusion of art and everyday life, sourcing drum patterns from the rhythms of train travel on the title track, “Metal on Metal,” and “Abzug,” and sampling the sound of breaking glass on “Showroom Dummies.” In mood, the album is torn between boundless optimism and darker, more doubtful tones. “Europe Endless” and “Trans-Europe Express” both speak to the dream of a unified European culture: Just 22 years after the end of World War II, what once had been an unimaginable utopia now seemed tantalizingly possible, thanks in part to the technological progress that powered Kraftwerk’s own synthesizers. At the same time, songs like “The Hall of Mirrors” and “Showroom Dummies” directly address the ambiguity between appearance and reality, suggesting that digitally enabled artifice might not be all it’s cracked up to be. Ultimately, Kraftwerk refuse to come down on one side or another; it’s up to the listener to decide whether the blissful vocoders of the closing track, “Endless Endless,” are genuinely hopeful or a cheeky glimpse at a post-human future.