Trestles Beach is the northernmost part of San Onofre State Beach and of San Diego County, but access is from San Clemente in Orange County. Surfers know this area near San Mateo Point as Uppers, Lowers, and Middles for the different surf spots near the train trestle the beach is named after. Unfortunately this surf area is fraught with localism which can be seen in the various spray-painted warnings along the path to the beach. Under the trestle at the beach San Mateo Creek stagnates and sometimes flows through the sand to the ocean. This wetlands area is called Trestles Wetland Natural Preserve and it is popular with local bird-watchers. It is a long walk down the trail from the parking area to the beach (about 1 mile each way).
The parking area is located near the intersection of S El Camino Real and Cristianitos Road east of Interstate 5 in San Clemente. A dirt path begins at this intersection and dips under I-5 on the way to the beach. Another path, which is flatter and paved, begins on the other side of I-5 and can be reached by walking over the bridge. San Mateo Campground is near the beach trail parking lot. The San Mateo area of San Onofre State Beach State Park has many hiking trails to explore.