In Robert Turner's book "The Thunder of Their Passing" he relates an accident on April 22, 1881 based on accounts in the Denver Daily Times.
The train was a westward mixed that had steamed out of the narrow confines of the Toltec Gorge. "The engine, freight cars and express car had passed along safely enough, ... when suddenly without warning of any kind the passenger car left the rails, and turning over, plunged down the embankment a distance of over 100 feet." One witness saw the car "bumping down in a most fearful manner..." and then hitting a tree, "there was a heavy crash and the coach flew into a thousand pieces." Apparently, heavy rains had softened the recently constructed roadbed, and the tracks gave way under the coach. Both couplers snapped as the coach toppled over, leaving the express car immediately ahead and the caboose just behind still on the tracks. When the horrified crew scrambled down the hillside and reached the crushed and splintered coach, they found seven passengers had been killed and five others badly injured. They helped or carried the injured and dazed passengers back to the train and placed them in the express car and caboose and rushed ahead to Osier.
The story continues in the book through the rest of the rescue and later investigation.
A search of the on line Colorado Historic Newspapers Collection did not find the specific newspapers/dates noted by Turner.
The Leadville Daily Herald of April 23, 1881 carried a story which stated the coach was number 11. This account said the accident occurred this side of Rock Tunnel on the Gunnison Extension, and the coach went 150 feet down the embankment. There were 14 passengers, six of which were killed outright, three with minor injuries and three with major injuries. No mention of the other two passengers. It also noted the coach was the only car that left the track.
Also in the Collection was a story in the Gunnison Review of April 30, 1881 which noted this was the story in the Denver Tribune of April 24, 1881. This was consistent with the other stories for the most part, but was late enough to include the names of the deceased and injured. This story had a total passenger count of 14, and claimed eight had died, but only listed seven names. They also noted all others had sustained injury, and listed the names of four people with the most serious injuries. The article also said those who were injured would surely recover.
I checked forward in time and did not find any accounts of an accident on that line from April 22, 1881 to the date in September of that year when the ticket agents had their memorial for Garfield, but did not have the presence of mind to check earlier than April 22, 1881 though. I did not find a story on the D&RG setting up a memorial marker noting the accident.
FWIW,
Dave Adams