At one point Gus tells Lorena that "If we both make it to Denver, I'll buy you a train ticket" (to San Francisco). She would have a long wait. The Denver and Rio Grande Western Railroad did not connect to Salt Lake City (and on through Ogden to California) for another 6 years.
"Lonesome Dove" is definitely the high-water mark of Mr. McMurtry's career, despite the few "anachronisms". Unfortunately, by the time he got around to writing "Dead Man's Walk", and "Comanche Moon", his "re-writing" of Texas history, made the novels impossible to follow, understand, and enjoy, especially to someone well versed in "Texian" history, and the American Civil War. In my opinion, he was trying to alleviate the suspicions that "Deets" was most likely a scout in the Confederate Cavalry (Oh! How politically INCORRECT....despite being based on historical FACT...i.e. the Sgt. Major in "Terry's Texas Rangers").