American Locomotive Company built 'poppet valve' Niagara #5500 - There was only one locomotive with this designation and was commonly referred to as the 'poppet valve' Niagara, it was the only Niagara to utilize Franklin poppet valves.  The S2a was mated with the only PT-6 class tender built, which created a totally unique configuration. The S2a has never been modeled in O scale.