Railroad Archives Online
- CollectionD0063 - Walter W. Ainsworth Collection
- Series007 - Spokane, Portland, and Seattle Railway
- Subseries Level 1014 - Photographs
- Subseries Level 2003 - Railroads--Buildings and Structures
- Subseries Level 3005 - Signals
- ItemWWASPSL4-072 - Spokane, Portland, and Seattle (SP&S) Signal Number 4.1 at North Dalles, Washington, undated