By Rail
Amtrak's Pacific Surfliner
runs from Paso Robles to San Diego, and Metrolink's Orange County Line runs from Los Angeles to Oceanside. The nearest stations to Knott's Berry Farm are in Buena Park itself serviced only by Metrolink and in nearby Fullerton serviced by both Amtrak and Metrolink.
By car
I-5 (Santa Ana Freeway)
or CA-91 Riverside Freeway takes you directly to Buena Park. Take the Knott Ave, Beach Blvd, or any of the other Buena Park exits. Anaheim is just south of Buena Park, with Fullerton to the east, so both of those cities can serve as an entry as well.