Bridge L-158 on the Lake Mahopac Branch is a Phoenix Truss, a truly unique design from Phoenix Iron Works of Phoenixville, PA. It uses tubular iron compression members (end posts, top chord and intermediate posts). They fit over cast iron lugs at the panel points.
The bridge was originally built as the main span of a double-track bridge for the New York, West Shore and Buffalo, over Rondout Creek at Wilbur (Kingston) New York about 1883. The New York Central moved it to Lincolndale in the 1900's, (the present West Shore bridge is dated 1905) and "narrowed" the trusses to a single-track bridge by shortening the floor beams and lateral systems.
I haven't seen a rating on the bridge, but considering that it was designed for loads on two tracks, and it only carried one track, it probably was not the load-limiting bridge on the branch. The F-12's on the Put carried old Class C tenders to limit their total weight, and I think that was for the bridge over Croton Reservoir. I also think I have seen a photo of an F-12 with its original heavy tender in service on a Harlem train that ran from Brewster via the branch and Golden's Bridge to North White Plains, but I wouldn't swear to that.
I know of one Phoenix truss remaining in railroad service, on an industrial side track on the Paducah and Louisville near Elizabethville, Kentucky.
Conrail had one on the former Reading RR Schuylkill and Lehigh Branch in Reading, PA, but it was neatly dispatched one day in 1976 by a 17 foot 3 inch high car of auto frames that tried to fit through its 15 foot 8 inch high portal. The frame car wound up draped over the center pier in the middle of the Schuylkill River. Those bridges are/were built of wrought iron, which is unpredictable as a structural material but very durable against corrosion.