Complex sentence: The construction of the "Pacific Coast Extension" of the Milwaukee Line occurred late in the historical era of railroad construction, extending from 1901 (when surveying began) through 1909 when a final golden spike was driven at Garrison, Montana.
Split into simpler sentences: The construction of the "Pacific Coast Extension" of the Milwaukee Line occurred late in the historical era of railroad building. The Pacific Extension surveying started in 1901, and continued until 1909 when a final golden spike was driven at Garrison, Montana.