Under the measure backed by 55% of voters in King, Snohomish and Pierce counties, $US 27.7bn will be raised over the next 25 years through a 0.5% sales tax (in addition to the existing 0.9% levy); a 0.8% car excise tax (based on the value of the vehicle) in addition to the existing 0.3% tax Sound Transit will collect until 2028; and a tax of $US 0.25 on each $US 1000 of the value of property.

ST3 will expand the light rail network five-fold to 187km, boosting ridership to around 600,000 passengers per day. Services will operate 20 hours a day with six-minute headways during the peak and 10 minutes at other times.

In addition to the four light rail projects planned for completion before 2022, the programme encompasses eight extensions:

- Redmond Technology Center - Downtown (2024)
- Kent/Des Moines - Federal Way (2024)
- Federal Way - Tacoma Dome (2030)
- West Seattle - Downtown Seattle (2030)
- Ballard - Downtown Seattle including new city centre LRT tunnel (2035)
- Lynnwood - Everett via Southwest Everett Industrial Center (2036)
- Tacoma Link extension to Tacoma Community College (2039), and
- South Kirkland - Issaquah (2041)

In addition to these extensions, new light rail stations at South Graham Street, South Boeing Access Road, and Northeast 130th Street are due to open by 2031.

ST3 also includes provision for investment in the Sounder commuter rail network. On the South Line from Seattle to Lakewood, where ridership has increased 15% in the last year, platforms will be lengthened to accommodate 10-car trains and services will be extended to Tillicum and DuPont. Track and signalling will also be upgraded to increase capacity.