The below information was gleaned from the Milwaukee Road email list at Yahoogroups.com.
The Royal City branch never came close to paying for it's construction.
The Traffic Department had made a very detailed study of this proposal
and found there was not going to be sufficient revenue to warrant the
expenditure of a spur and to support the expenses of spur operation.
However Josh Green was opening a bank in Basin City and sold Vice-
President Lutterman the need for a Royal Slope branch.
In the early 1980's, after the demise of the Pacific Coast Extension, this trackage was operated by BN, as evidenced by the following Portland Division ETTs:
1982 BN 20th Sub, Warden to Othello, 12.8 miles
1983 BN 13th Sub, Warden to Royal City, 38.9 miles
1984 BN 10th Sub, Warden to Royal City, 38.9 miles
1986 BN 9th Sub, Warden to Royal City, 38.0 miles
Interesting to not the diminishing number of the subdivision, as BN abandoned others. Not sure exactly what happened between 1980 and 1982 -- I need to check. But by October 1986, the line was not listed in the BN ETTs -- the line was transferred to Washington Central.