The Milwaukee Road line from Tacoma to Chehalis is miles longer than the ex - NP Ft. Lewis line, running as it does due South UP the Gulch to Midland and Frederickson, then West / Southwest to Yelm & Maytown, then due South again along Interstate 5 to Blakeslee Jct, Centralia, and Chehalis. This line was built by predecessor railroad Tacoma Eastern on the cheap, which was also the Milwaukee Road way. Maintenance was non - existent in later days; the line also has many dips and curves that would keep train speeds low, unless realignments were made.
If anything, the old NP Prairie Line from Lakewood South through East Ft.Lewis, Roy, Yelm and Rainier to Tenino would be a better option.