Railroad Archives Online
- CollectionE0316 - Robert W. Downing Collection
- Series004 - Northern Pacific Railroad Company
- Subseries Level 1014 - Photographs
- Subseries Level 2001 - Railroads--Equipment and Supplies
- Subseries Level 3001 - Steam locomotives
- ItemJSRH60840 - Northern Pacific steam locomotive Class Z-6 pulling a freight train.