The Rio Grande had at least two rectangular tanks.
One was located at Rogers, above Crested Butte on the Floresta branch, measured 7'x30x8'6" with a peaked roof. At least one photo of this tank has been published.
The other was at Mill Creek, on the west side of Marshall Pass, between the summit and Tank 7. This tank measured 10'6"x21'x9', presumably with a flat roof. This tank was destroyed by fire May 26, 1927, retired AFE 3409-1927.
This information, along with the answers to many of the questions asked on this forem, can be found at the Colorado Railroad Museum Library.
Tom