Numbers 1506, 1503, and 1500 in work train service at Denali Park in 1979. Matched A-B-A sets of black and yellow F-7's were common in work train service in the mid to late 1970's. The 1506 was from the second set of F units delivered to the ARR and has the normal fuel tank. If you look closely you can see the extended fuel tanks on the 1503 and 1500 which came on the first order of ARR F-7's.
 

Photo courtesy of Jason Kuehn