I'm out and about right now so I can't look it up, but most likely during the 50s. The United States transitioned away from steam in the 50s and was largely transitioned to diesel by the early-mid-60s

Edit: this xitter xeet says 1957 which seems very plausible (and applied a filter to the photo for whatever reason) Also while reverse image searching in hopes of finding more details I found this very post but federated to dbzer0.com