History/Tech Question

I recently spent some time walking and photographing the old South Park line from Morrison to Leadville. On the grade out of Como toward Red Hill I came across a cast iron rail brace - doo-widgit with "No.4" embossed in fancy block letters on it. It has 3 square holes for spikes and the opposing end has a negative profile of a very light rail. Looks to be an outside curve reinforcement brace. The question is: How old is this? The rail dimension looks to be lighter than any I have ever seen in use, and I can almost hear Mason Bogies running past if I sit quietly enough. At what time/s were/was the South Park rail replaced with heavier stuff, and what weight rails did they use over the years? Thanks in advance!