The first was 2.63 miles (4.23 km) in length and opened in 1900 to avoid problems caused by heavy winter snowfalls on the original line that had eight zig zags (switchbacks). The current tunnel is a 7.8 miles (12.6 km) in length and entered service in early 1929, approximately 1.5 miles (2.4 km) south of and 500 feet (150 m) lower in elevation than the original. The present east portal is nearly four miles (6.5 km) east of the original's and is at 2,881 feet (878 m) above sea level, 1,180 feet (360 m) below the pass. The tunnel connects Berne in Chelan County on its east with Scenic Hot Springs in King County on its west and is the longest railroad tunnel in the United States.
The first tunnel began construction on August 20, 1897, and was completed on December 20, 1900. The tunnel was 2.6 miles long. John Frank Stevens was the principal engineer on the interim switchback route (opened in 1893, with grades up to 4 percent) and the first Cascade Tunnel. Stevens Pass, located above the tunnels, was named after him.
The tunnel had a fume problem from the coal-burning steam locomotives. It was built with a 1.7% (1:58.8) gradient eastbound, which was too close to the ruling gradient of 2.2%. Because of the steepness of the line, the locomotives had to pull hard to make the grade and thus burn more coal, which would lead to immense smoke in the bore. The tunnel was electrified, with the project completed on July 10, 1909, eliminating the problem. The unusual system used was three-phase AC, 6600 volts at 25 Hz, from a 5 MW hydroelectric plant on the Wenatchee River just west of Leavenworth. The tunnel section only was electrified; 4.0 route miles (6.4 km) or 6.0 track miles (9.6 km) and 1.7 percent grade through the tunnel.
The motive power for the section consisted of four GN boxcab locomotives supplied by the American Locomotive Company; they used electrical equipment from General Electric and were of 1500 hp and weighed 115 short tons (104 t) each. Initially three locomotives were coupled together and hauled trains at a constant speed of 15.7 mph (25.3 km/h), but when larger trains required four locomotives the motors were concatenated (cascade control), so that the speed was halved to 7.8 mph (12.6 km/h) to avoid overloading the power supply.
The tunnel was still plagued by snow slides in the area. On March 1, 1910, an avalanche at Wellington (renamed "Tye" after the disaster), near the west portal of the original 2.6 miles (4.2 km) Cascade Tunnel, killed 96-101 people, the deadliest avalanche disaster in U.S. history. This disaster prompted the construction of the current tunnel.