Railroads meant more horses.
http://www.uctc.net/access/30/Access%2030%20-%2002%20-%20Horse%20Power.pdf
A very intetesting article about urban draft horses and urban areas in the 19th and early 20th Centuries. It is mentioned that for a while railroads meant more horses and not less, due to the greater need for distribution of rail hauled goods. I ran across one date of 1915 as the peak of horse population in the US. This is a date very close to the date of peak rail passenger miles, which was around 1912.