The railroad at the zoo opened in 1968 and it’s a replica of the Union Pacific 119, a train dating back to 1868 so it marked a useful centenary milestone. Although I didn’t see this replica, I only saw their other steam locomotive, which is Riva. This train is Austrian and was made in 1890 when it was used on an industrial railway.
When Union Pacific closed their maintenance shed in the 1990s they gave a lot of their equipment to the zoo to help keep everything working smoothly. Over 200,000 people ride this train every year, which is an additional charge to the zoo entrance fee.