I want to know the answer to the following question:
How long did three-phase system operate in Cascade Tunnel?
All the background I'm provided with is that "Other lines where the three-phase system were used were the Simplon Tunnel in Switzerland from 1906 to 1930, and the Cascade Tunnel of the Great Northern Railway in the United States from 1909 to 1927.". Is that enough to answer the question?
Answer: yes