The subject line is correct - these are not "drover's cabooses" any more than the Pagosa Jct. combine was. These were used to provide mixed train service on lightly traveled lines. The cupola provided an advantage compared to just assigning a regular combine.
http://en.wikipedia.org/wiki/Alamogordo_and_Sacramento_Mountain_Railway
http://www.foresthistory.org/ASPNET/Publications/region/3/lincoln/cultres4/sec3.htm
This line caught my eye a while back as due to its unique nature. Some of the small 2-8-0s used by the SP on this line were transferred to Oregon for their last few years to supplement the remaining light 4-8-0s until GE 70 tons replaced them.
I have seen a builder's photograph of one of the smaller combination cabooses. Caboose M-1 has a side door, cupola and three consecutive passenger(?) windows shoehorned onto a short frame. The lettering cast into the wheels is highlighted showing it to be a St. Charles Car product. It does not have the US Mail, baggage & Express lettering on it suggesting the long caboose might have provided the regular passenger service. It wasn't that unusual for small railroads to have the formal or informal practice or carrying passengers on a caboose making all freight trains in effect mixed trains.