Pony trap gauge

This image is sliced from a larger one I stumbled across at Shorpy. The caption states: New Orleans circa 1890. "St. Charles Hotel from Canal Street." 5x7 inch glass negative by William Henry Jackson.

I was intrigued by the pony trap rolling down the street in the left-hand set of tracks. Its wheels, although a little skewed, are rolling in the street track's groove. The ride must have been a vast improvement compared to rolling over the cobblestone street. The picture gives a lot of insight into how the gauge for horse drawn trams, and later their electrified descendants, were decided upon.