While not exactly "mixed trains", the D&RGW did haul revenue passengers in cabooses. According to Employees Timetable 121 dated June 11, 1939,
Special Rule 7: Persons accompanying live stock or other frieght whaen holding proper transporation, and when permission to accompany same is covered by contract. Passengers on freight trains should be informed that cabooses will not be pulled up to platform to recieve or deliver passengers or baggage. Employees holding passes will be carried on any freight train to and from points at which trains stop are stamped: "Good on Freight Trains".
Special Rule 7-A: "Passengers may be carried on Extra freight trains between Salida and Alamosa, on Creede Branch and bewteen Alamosa and Antonito and between Chama and Durango."
Interesting that you couldn't ride between Antonito and Chama....