Shown in this image is the horse drawn Main Street trolley, which ran on the rails just visible below and in front of the horse, from the main entrance to the central plaza of the park.
The description states:
Dobbin stands patiently while the conductor of Main Street’s trolley counts his fares before heading to the Plaza at the other end of Disneyland’s Main Street.
–This attraction has been maintained and is still a popular activity along the original route. This image dates from shortly after the park opened in 1955.