Interesting ask, Greg!
But one-track album, or one-track sides?
In-A-Gadda-Da-Vida (the track) was side two of In-A-Gadda-Da-Vida (the album)
If one-track sides: "Echoes", side 2 of Meddle - Pink Floyd
If one-track albums: "Thick as a Brick" - Jethro Tull (Yes, I reckon it can be argued that the track does continue from end of side one to side 2. I just checked - Wikipedia agrees.)
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