Despite what the signs on the fence say, you can ride your bike or walk across the tracks to get to the Chino Hills State Park, which you can access via the Skully Ridge or Aliso Creek trails, both of which branch off the gravel road that crosses the tracks at this point. The golf course and the railroad are just trying to discourage random cars and golf carts from coming through, is all. Golfers have no business down there, and the railroad reserves the right to park trains here for hours at a time. They don't want frustrated people to do anything stupid like crawl under a train to get to the other side. You might just have to wait.
None of that applies to this cache, though. You don't have to cross the tracks to hunt for this one.
Don't park in the golf course lot and walk down to it unless you're a customer, a status you could earn by hitting a bucket of balls, visiting the pro shop or grabbing a bite at the 19th hole. If you're hiking or biking into CHSP you should be parking in the SART bike path parking lot back near the freeway.