Robert,
According to Chappell, et al in
Colorado Rail Annual 12, the UP transferred the Rhode Islands from the U&N to the DSP&P, the latter line receiving the locomotives in September of 1890. That would provide an "earliest date" for this photo of Utah & Northern 261, shown on a northbound freight at Wheeler (later Solitude):
In CRA 12, page 177.By this c.1890 photo, number 261 (later C&S number 58) has had its smoke box shortened, has had a standard UP diamond stack installed and is still lettered on the cab for "Utah & Northern". Note it has the small extended smoke box door, with 4 "dogs" that survived to the modern era only on C&S 60. So, it is likely that the U&N shortened the smoke boxes some time prior to 1890.
The only U&N photo of a Rhode Island that I've ever seen is the builder's photo with the straight stack and long extended smoke box. I've never seen a photo of a "Rhodie" in actual service on the U&N.
As the Rhode Islands were delivered in 1886 and the UP was standardizing locomotive appliances on all its narrow gauge subsidiaries, I would speculate that the smoke box was shortened shortly after delivery and the UP diamond stack was also installed at that time. I doubt that the locomotives ever sported a Congdon stack, as by 1886 they were pretty much disappearing on the South Park and Central locomotives, in favor of the UP diamond stack.
Jim Courtney
Poulsbo, WA