[10][11] The N Line begins in Seattle and travels north for 34.2 miles (55.0 km) on the BNSF Scenic Subdivision towards Snohomish County, where it serves three stations and terminates in Everett.

[15] The N Line continues northwest past Myrtle Edwards Park and under the Magnolia Bridge to traverse the Balmer Yard, a major BNSF railyard in the Interbay neighborhood.

[26][27] The S Line is 47.6 miles (76.6 km) long and travels south along the State Route 167 corridor from Seattle to Pierce County, where trips terminate in either Tacoma or Lakewood.

[1][28] Trains begin at King Street Station and travel south along the east side of Lumen Field and T-Mobile Park, passing under the retractable roof of the latter, in the industrial SoDo neighborhood.

The tracks pass an auxiliary railyard for Sounder trains and turn southwest near McChord Field to reach the southern terminus of the S Line at Lakewood station.

[9][23] The first railroad to reach the Puget Sound region was the Northern Pacific Railway, a transcontinental route that began construction with a section from the Columbia River at Kalama in May 1871.

[47] Several communities on Puget Sound competed to become the terminus of the Northern Pacific and offered land and other incentives; Tacoma on Commencement Bay was chosen in July 1873 ahead of Seattle and Olympia.

[48] The railroad to Tacoma was completed in December to meet a deadline imposed by the United States Congress and scheduled passenger and freight service began on January 5, 1874.

[48] It began passenger service from Seattle to Renton in 1877 and was reorganized as the Puget Sound Shore Railroad as it was extended south to Stuck Junction near modern-day Auburn in 1883.

[50][51] Northern Pacific briefly ran passenger trains between Tacoma and Seattle from July to August 1884 on the Puget Sound Shore Railroad, which had been upgraded to standard gauge but lacked track ballast.

[58] Both railroads operated daily local passenger trains in the Puget Sound region, including a dozen trips to Tacoma and eight to Everett by 1911; most had onward intercity service to Portland, Oregon, and Vancouver, British Columbia.

[59] A set of electric interurban railways were operated by private utility Stone & Webster in the early 20th century to provide more frequent passenger service from Everett and Tacoma to Seattle.

[60][61] The Puget Sound Electric Railway from Seattle to Tacoma began service on September 25, 1902, with 22 stops on local trains and express runs that reached 60 miles per hour (97 km/h);[61] it had a peak ridership of 3 million in 1919.

[61][63] Both services were faster than other trains and the "mosquito fleet" of steamships on Puget Sound, but were not profitable and faced increasing competition from automobiles and buses on the expanding highway system.

[79] Burlington Northern agreed to share technical information with Metro Transit for their studies and stated that they were interested in operating the trains, which they could accommodate with the construction of a parallel track.

[84] Burlington Northern commissioned a study for a Seattle–Everett commuter rail line in late 1992 after discussions with local governments and Community Transit, the bus operator in Snohomish County.

The Central Puget Sound Regional Transit Authority (RTA) was approved by the King, Pierce, and Snohomish county councils and was formally created in September 1993.

[94] It used a set of 14 bilevel cars, each able to carry 150 passengers, from GO Transit in Toronto that had previously been leased to Metrolink in Southern California to provide additional service after the 1994 Northridge earthquake.

[97] The demonstration was delayed due to issues securing financing from the state settlement and was renamed to reflect its expanded scope, which included rush hour service to Seattle that began from Everett on January 28.

[105] Amid discussions of a second RTA ballot measure, King County Executive Gary Locke proposed a smaller package that excluded the commuter rail system in favor of funding highway improvements.

[108] The RTA restored service to Lakewood in the final plan adopted in May, which allocated $669 million to the commuter rail system with fifteen daily trains during peak hours.

[111][112] The RTA's preliminary schedule for the projects in Sound Move was adopted early the following year with plans to begin construction on commuter rail stations in 1998.

[121][122] The launch of service to Tacoma was delayed by several months due to negotiations with Burlington Northern Santa Fe, who sought state funding for track improvements and other projects.

[135] Sounder generally serves rush hour commuters and runs almost entirely on weekdays, with most trains traveling inbound to Seattle in the morning and outbound to the suburbs in the afternoon and evening.

[141] Sound Transit contracts with BNSF Railway for dispatching and staffing of conductors and engineers who operate Sounder trains,[9] which primarily use tracks owned by the company.

[150][154] Sounder fares on the N Line corridor are able to be used on select weekday Amtrak Cascades trains serving Everett, Edmonds, and Seattle as part of the RailPlus program.

[179][180] Ridership is measured by on-board infrared automated passenger counters that are installed on every Sounder car;[1] they record the number of people entering and leaving the train.

[197][198] Caltrain in the San Francisco Bay Area purchased 5 locomotives and 17 passenger cars in 2001 after Sound Transit delayed the launch of additional round-trips on Sounder.

[28] Plans to extend the existing station platforms to accommodate ten-car trains, a capacity increase of 40 percent from the seven-car maximum, were originally scheduled to be completed by 2028.

[208] The extension would the existing Point Defiance Bypass constructed by Sound Transit, with a second track added in some sections,[209] and generally follow the Interstate 5 corridor near Joint Base Lewis–McChord.