It struck me as anachronistic that the 1915 worlds fair would demonstrate the steam locomotive, a technology that had been around for decades.

Reality turns out to be more interesting, it was an exhibit of Southern Pacific's first steam locomotive, the CP Huntington, built in 1863.

So while it's true to say that the 1915 worlds' fair exhibited Southern Pacific's first steam locomotive, it's also very misleading to suggest it was new technology of the time.

~85 years prior to the 1915 fair, Stephenson's Planet 2-2-0 steam loco was operating commercially between Manchester and Liverpool (5 built in 1830). The planet was a version 2 of his earlier Rocket already operating between the cities since 1829/30 - utilizing a very design with the boiler between the wheels layout that almost all steam locomotives eventually adopted.