Check out this truly EXCELLENT website discussion board focusing on Southern California. This should be your very best way to answer your question from the source.
Several members are very active in the San Timoteo Canyon and Cajon Pass areas:
www.socalrailfan.com
www.socalrailfan.com/forums/index.php
--- Daniel
JOHNKLUFT wrote:HELLO, I AM INTERESTED IN RAILFANNING THE UP ALONG IN SAN TIMOTEO CANYON, I HEAR IT IS A VERY INTERESTING LOCATION. CAN ANYONE HELP WITH DIRECTIONS, HIGHWAY EXITS, GOOD PHOTO SPOTS ETC. ALSO, I WILL BE COMING FROM PALM SPRINGS AREA, (ON VACATION FROM NEW YORK) CAN I DO THIS ON THE WAY TO CAJON PASS? ANY SUGGESTIONS WILL BE APPRECIATED. THANKS, JOHN KLUFT