The final destination for Buck on the train in which he was placed in a cage was Seattle. It appears that the train itself might have continued on after the express messengers "bundled (Buck) off the train" in this city", but its final destination is not clear in the book.
After Buck was stolen from Judge Miller's ranch, he was beaten, choked and thrown in a semiconscious state into a baggage car of another train by an unidentified man who bought him for a hundred dollars. This part of Buck's journey was comparatively brief, beginning at a "little flag station known as College Park" in California's Santa Clara Valley and ending after a night in San Francisco. Buck was then transferred to "a cagelike crate", which proceeded to make "a passage through many hands", until it was finally deposited in an express car". Buck travelled "for two days and nights" in this express car "dragged along at the tail of shrieking locomotives", and for all that time, he "neither ate nor drank". At long last, Buck's train ride ended when, in his cage, he was unloaded at Seattle. In Seattle he was released from the cage by a brutal dog-breaker, who quickly taught Buck that "a man with a club was a lawgiver, a master to be obeyed" (Chapter 1).