That 'exact date' came in 1974...
"Road and yard electrification ended on June 15-16 at Deer Lodge, Montana. Train 264 arrived that Saturday at 7:50 a.m. There were no media people present, but a few faithful railfans, including well-known Richard Steinheimer, had traveled long distances to witness the final arrival. A diminutive yard switcher was the final electric locomotion, working the Deer Lodge yard until early hours the next day. Milwaukee electrification ended the same way it had started more than 58 years earlier, using a General Electric steeple-cab goat."
The Milwaukee Road 1928-1985
by Jim Scribbins
© 2001 Heimburger House Publishing Company