I had to look up the stats on YW 8/9. What I have shows 51" drivers with 20" by 24" cylinders giving a t.e. of 28,000 lbs.. McCloud 25 has 46" drivers with 19" by 24" cylinders giving a t.e. of 28,800 lbs., so comparing the two for paper pulling power is a wash.
McCloud 20-23 were the smaller versions with 17" bore cylinders so bigger sisters 24 & 25 were no slouches as pullers even though ordered for logging line duty. I was told just a couple of years ago by a McCloud engineer that 25 was rated for more cars out of the McCloud depot than bigger sister 18...and her t.e. was something over 35,000 lbs. The point that I'm trying to make here is that YW would have known when they requested 25 for duty, that she would have been a comparable and satisfactory locomotive as a leased spare.
Still feel that event did occur, there might have been other circumstances such as 8 not being available right at the time a spare was needed. These things will happen when you are operating machinery.
Appreciate all the YW info that has been submitted here, thanks.