One observation about why coaches ended up on the White Pass.& Yukon Route.
The best I can tell is that the White Pass & Yukon Route had an almost steady need for more passenger cars due to its tourist business. But had an adequate supply of freight cars
From what I've seen the WP&YR was promoting itself to tourist by 1900.
In 1916 it was buying cars probably because because of an increase in business from the people who would have traveled to Europe in earlier years. Who wants to take a vacation into a war zone! The WWI period saw the American railroads promoting travel to vacations spots in the West and the WP&YR did the same. In the late 20s the railroad was seeing increase in passenger business probably because the economy was doing well more people thought about taking vacations -- so it was searching for and buying passenger cars. Then as things were getting better in the late 30s for some of the people once again travel to Alaska and the Yukon picked up.
So it bought equipment from the N-C-O in 1916, Then cars from the NWP and from one of the lines at Morenci Arizona. And from the Pacific Coast. Then after WW2 the coaches from the Sumter Valley. Whatever it could find at a given time.
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