day, with the wind out of the west, turbulence usually starts a few miles west of Rialto and continues a few miles to the east, growing in strength above the altitude of the mountains and especially over the pass near the HITOP intersection. In Santa Ana conditions, up- and downdrafts can become violent northeast of Ontario Airport, and turbulence can be experienced east to the Banning Pass, well known for turbulence. The mass and wing loading of an aircraft determine its sensitivity to turbulence, so what may seem violent in a Cessna 172 may seem only mild to moderate in a Boeing 747. In the 2006 Mercy Air 2 accident, an air ambulance helicopter collided with mountainous terrain near the pass in foggy weather. Rail transport Traffic The California Southern Railroad, a subsidiary of the Atchison, Topeka and Santa Fe Railway, was the first railroad through Cajon Pass. The line through the pass was built in the early 1880s to connect the present day cities of Barstow and San Diego. Today the Union Pacific Railroad and BNSF Railway (the successor to the Santa Fe) use the pass to reach Los Angeles and San Bernardino as part of the Southern Transcon. Due to the many trains, scenery and easy access, it is a popular location for railfans, and many photographs of trains on Cajon Pass appear in books and magazines. The Union Pacific Railroad owns one track through the pass, on the previous Southern Pacific Railroad Palmdale cutoff, opened in 1967. The BNSF Railway owns two tracks and began to operate a third main track in the summer of 2008. The railroads share track rights through the pass ever since the Union Pacific gained track rights on the Santa Fe portion negotiated under the original Los Angeles and Salt Lake Railroad. The original BNSF (ATSF) line was built in the 1880s and later roads, U.S. Route 66 and I-15, roughly followed this route. The 3.0% grade for a few miles on the south track is challenging for long trains, making the westbound descent dangerous, as a runaway can occur if the engineer is not careful in handling the brakes. The second track, built in 1913, is longer to get a lower 2.2% grade. It ran through two short tunnels, but both were removed when the third main track was added next to the 1913 line. Trains may be seen traveling at