ATHG97971 Union Pacific UP 4-6-6-4 Challenger Steam Locomotive #3985Union Pacific In the spring of 1941 UP locomotive designers began working on a locomotive to team up with the new 4 8 8 4 Big Boy's then under construction at Alco. The Big Boy's were designed to pull heavy freights up the Wasatch Range in Utah to the division point at Green River, WY. What was needed was a locomotive to take over at that point that could speed the freights eastward across the Wyoming Division. Building on their experience with the