Spokane International Railway operated four ex-UP CA-3 steel cabooses, C-18 to C-21. Cabooses C-18 and C-19 were purchased in June 1962. Cabooses C-20 and C-21 were purchased in July 1963. Research suggests that these four steel cabooses operated only on the Spokane International, except as they were being moved around after being retired. They were retired in preparation for cabooseless operations on UP's former Spokane International lines.
Union Pacific acquired control of the Spokane International Railroad on October 6, 1958. Spokane International Railroad was formally merged into Union Pacific Railroad on December 31, 1987. UP originally held about 99 percent of SI stock, and gradually acquired the remainder. In 1959, Spokane International's equipment (including the 11 RS-1s and four remaining cabooses) was first leased, to UP, then later actual ownership passed to UP.
The Oregon Public Utilities Commission granted a waiver to UP allowing cabooseless operations in early September 1985. Following the waiver, the first cabooseless mainline train on Northwestern District was on September 8, 1985, from Hinkle to Spokane, bound for interchange with Canadian Pacific. The same train initiated cabooseless operations over the former Spokane International on the same day, between Spokane and the Canadian Pacific interchange at Eastport, Idaho.
From Utah Rails,