It would appear that the horse is set to push the sightseeing car. More likely, he is just turned around so he can follow behind on the trip downhill. If so, I hope there's also a brake.
I wouldn't have a clue to where this is, except for the sign below. Here it is magnified several times.
It says, "To Echo Mountain via Castle Canyon 34 miles." That should give a good clue. Echo Mountain is located in Los Angeles County in the San Gabriel Mountains. It's called Echo Mountain because of the great echoes you get when you yell into Castle Canyon. And there was a railway there too, the Mount Lowe Railway, which was powered by overhead trolley wires and transported passengers from Altadena up the hills to Rubio Canyon, where there was a small hotel. From there, passengers could take a funicular up to Echo Mountain, where there was an elegant 70-room Victorian hotel and a chalet. There was also a casino, a dance hall, and the opportunity to transfer to yet another trolley line to Crystal Springs. The entire venture constantly ran into hard times and disasters, including fires, torrential rains, and gale-force winds. It opened in 1893 and was abandoned in 1938.
This horse-drawn car may have connected to the Mount Lowe Railway, but I don't think it was part of the system. The former owners of this postcard were thoughtful enough to write a date on the back though. These were the same people who visited Gayle's Lion Farm and nicely dated those cards as well.
Update 3/8/11: I happened to see a very similar card today taken at the same location. The people who took that picture wrote on the back: Mule train, Inspiration Point to Picture Rock, Mt. Lowe.
Here's the back of the card.