I lived in California for several years, many in southern California. These trees in the picture look like the trees in and around the Joshua Tree National Park. The National Park was created in 1936; the picture on this postcard is from at least 35 years prior to that. Notice that it takes two steam engines to pull only 6 cars. If I remember right (the last time I was there was probably in 1976 or 77) the park is in California's high desert area north of Palm Spring and Indio.
The copyright on the front is from 1901 by the Detroit Photographic Company.
This company was started in the 1890s in Pearl River, New York. It changed its name to Detroit Publishing in 1905. That would mean that this postcard was printed sometime between 1901 - the date of the copyright and 1905 - the date when the company changed its name.
Another clue to the date of the postcard can be found the on the reverse side. There is no room for a message from the sender on the back. I have seen messages scrawled around the pictures on the front of the card, but, no writing was allowed on the back side except the address.
That means that this card was printed sometime before March 1, 1907. That was the date when the United States Post Office grudgingly allowed the address to share space with a short message on the back of a picture postcard.
They very quickly learned what a great decision that was and probably kicked themselves for not going forward much sooner. Many companies started using the picture post cards for advertising and millions of tourists sent them to their friends and families to show where they had been.