In the vinyl era one-track albums were very rare. I can think of Thick as a Brick and Passion Play (both Jethro Tull). Tales from Topographic Oceans (Yes) was four one-side pieces that were musically and lyrically different but conceived as being related 'thematically', but how you could ever work that out from the lyrics has me baffled (nice album though). In the CD era I think there have been a few. There have certainly been some 'ambient' albums. There are also those that have distinct movements but which play continuously (eg Pat Metheny's This Way Up which is subdivided into Opening Part 2, Part 3 Part 4).
One-track sides were more common. In-A-Gadda-Da-Vida (1968) was probably the first. I used to think Echos (1971) was second but the folk group Pentangle did Jack Orion (on Cruel Sister) a year earlier. After that they were almost common. Yes were very fond of them: Close to the Edge, all four sides of Tales and The Gates of Delirium from Relayer. Genesis all but did one (Suppers Ready), both sides of Fripp and Eno's first album and the second side of the second, Eno's Discrete Music (and in the CD era Thursday Afternoon), A Plague of Lighthouse Keepers (from Pawn Hearts by van der Graff Generator) come to mind. There must be more.
And there is!!
Frank Zappa - Lumpy Gravy (1967) Parts 1 & 2 now appears to be the first example of one-track sides and one-piece album. (How strange for Zappa to be seminal.

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Other examples include:
Miles Davis - *****es Brew (1969) Pharaoh's Dance and *****es Brew
Miles Davis - In a Silent Way (1969) Shhh/Peaceful and In a Silent Way/Its About That Time
Soft Machine - Fourth (1971) second side is Virtually Parts 1-4
McDonald & Giles (1971) second side is the Birdman suite
New Trolls - Concerto Grosso (1971) - Nella sala vuota, improvvisazioni dei New Trol
Deep Purple - Made in Japan (1972) - Space Truckin'
Mahavishnu Orchestra - Between Nothingness and Eternity (1973) Dream
Jade Warrior - Waves (1975) Parts 1 & 2
Tangerine Dream - Ricochet (1975) - Parts 1 & 2
Vangelis - Heaven and Hell (1975) Parts 1 & 2
Led Zepplelin - The Song Remains the Same (1976) Dazed and Confused
Vangelis - Chariots of Fire (1981) (title track)
Vangelis - Soil Festivities (1984) Movements 1-5