The Tehachapi Loop is a .73 mile spiral that runs through the Union Pacific Railroad line that passes through Tehachapi Pass. Building began in 1874 by the Southern Pacific Railroad to ease the grade over Tehachapi Pass. The chief engineer William Hood designed it to rise at a steady 2% rate, reaching a maximum height of 77 ft (23 m) and a train more than 4,000 ft (1,200 m) in length will pass over itself when it travels on the track. The Loop became property of the Union Pacific Railroad after it absorbed the Southern Pacific Railroad in 1996. Union Pacific bars passenger trains from the line, which prevents Amtrak's San Joaquin train from serving Los Angeles (passengers instead must board Amtrak Thruway Motorcoaches to connect from Bakersfield to Los Angeles). This has been the case since the creation of Amtrak in 1971. An exception is made for the Coast Starlight, which uses the line as a detour if its normal route is closed.