That is a RR tie plate, looks to be a 5 1/2" plate, the 5 1/2" being for smaller rail, the measure of the base of the rail, say rail being 90lb to 115lb rail, (the weight of the rail for every 3'. Standard rail of say 160lb has a 6" base. The cap looks to be from a welding tank. Neither would be a surprise since it seems to be a rail road trestle. Surprised there were no bent spikes.