Manually, from Hereford & Robart - Rio Grande Narrow Gauge - The Final Years, Alamosa to Chama, page 6.

"On December 5, 1968, the last Rio Grande train over Cumbres departed Alamosa for Durango.
Powered by locomotives 483 and 473, it comprised locomotive 481 (not used after it had been overhauled two years earlier), a gondola of boiler flues, three Silverton branch passenger cars and three cabooses. Nine empty boxcars were included to provide additional brakes for easing the train down the west side of Cumbres Pass. After spending the night in Chama, 473 took the train west, reaching Durango late in the evening of December 6. 483 returned light to Alamosa."

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D&RGW operated the Durango - Silverton for awhile, so readers may now research the transfer to the non-common carrier operator.

The National Park Service ran an evaluation tour, between December of 1968 and July 14, 1969 - final abandonment by the ICC.

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So, there can be three "lasts". Take your choice.

One of the three locomotives noted above is on the Cumbres & Toltec Scenic Railroad roster, available in the Friends website.