The huge end log supports, compared to the track & particularly the skinny rail, almost look 'wrong'.
I wonder why the center section was made as it was ?
To let the creek flow though unobstructed.
Two pig-sty piers on solid ground, the 2 horizontal support beams to carry the supports to the centre of the span between the pig-sty piers that are doing most of the work. Simple backwoods logging engineering 101 - probably built that way after a central pier was washed out??
Provided the creek doesn't get high enough to bring down heavy debris that snags those horizontal support beams, it'll last for years.
Probably good for about 10 ton axle-loading, so Shays, etc to about 40 tons all up. As to gauge?? Might be less than standard - say 3'6" or so?? if 3'6", then possibly Kiwi?
Unanderra in oz