This 703 was actually the second F3B BN had with that number. The first 703 was ex-NP F3B 6001C but it was retired in 1974. The 2nd BN 703 is ex-GN F3B 353B.
Built in November of 1947 for the Great Northern as F3B 353B it probably worked passenger trains most of it's time while on the GN. Originally delivered in the classic Empire Builder scheme of orange and green it also had silver trucks for awhile. It was later painted in the simplified GN orange and green scheme.
I saw the GN 353B just once before the BN merger. It was between runs at Interbay on 2-15-70.
After the BN merger day it continued to run on BN passenger trains and was renumbered BN 9707. For example I saw it 12-26-70 on train #32 the Empire Builder leaving Seattle at 3:45pm with the BN 9852/9707/9703 (SDP40/F3B/F3B) for Stevens Pass.
In looking over my notes I see that former GN passenger power tended to work the ex-GN lines and former NP passenger power was common on the ex-NP.
After Amtrak took over passenger train operations in 1971 the 9707 could be found on the ex-NP as well. Here are some examples of F3B 9707 in early Amtrak service:
11-13-71 #32 9772/9708/9707/9726 Easton, WA (#32 now on the ex-NP)
11-20-71 #8 9772/9726/9707/9749 Auburn, WA (#32 renumbered to #8)
01-29-72 #8 9766/9707/9745/9724 Easton, WA
Once Amtrak received new power the ex-BN F-units were released to freight service on the BN. In 1975 the 9707 was assigned to Auburn, WA where it was used in all kinds of service out of Auburn including over Stampede Pass.
Here is a photo of the BN 9707 at South Seattle, WA on 5-12-75.
And another of the 9707 at Auburn, WA 1-18-76.
Info shows the BN 9707 was renumbered BN 703 and painted green in 1976. It continued to be assigned to Auburn, WA as the 703.
Here is a photo of the 703 at Interbay, WA 8-6-77 wearing BN green paint.
Another photo of the 703, this time at Auburn, WA 7-29-79 with a bit of the ex-GN orange showing around the middle porthole.
And one last photo of the 703 at Interbay, WA on 6-30-80 still looking pretty good for a locomotive built in 1947!