Denver had horse-drawn streetcars in 1871. It was also one of the few cities that tried cable cars before converting to electric streetcars. Five cable car lines were built in 1888 and 1889, but since there were no major grades in Denver, they were soon replaced with the more efficient electric-powered streetcars.
The College of Architecture and Planning at the University of Colorado has put together a nice website with information on Denver's streetcar history and new streetcar developments, so rather than repeat the information, I would refer you to their site.
Here are the backs of the cards in the same order.
Dear Mother: Your welcome letter just rec'd - Mine giving all information must have reached you ere this. Wrote soon as I found out. The Elks are here, going to parade now. Snow on streets here for the event brought from mts. - Love BessJuly 15