This is another example of fans being confused by the over exposure of photos of specific trains, in this case excursions, on specific days in time.
No, the Rio Grande Southern did not run passenger trains with cabooses, or gondolas, in regular service. In fact, after 1932 or so there were no RGS passenger trains but before that date they either ran all passenger cars or passenger cars and freight cars in mixed train service, without cabooses.
What you are seeing are Rocky Mountain Railroad Club excursions in 1947, '49 and '51. In each case a caboose or two was added to the available passenger cars and hastily converted open gondolas to make up an excursion consist.
They were three trains on three specific Memorial Day and Labor Day weekends in history and account for an incredible amount of photographic "evidence" because of all the camera wielding fans.