Steam locomotive profile: 4-6-6-4 Challenger | Classic Trains Magazine,The 4-6-6-4 ,4-6-6-4 Steam Engine UP Challenger #3977 at the Randall Museum Model Railroad,Most successful articulated locomotive: The 4-6-6-4 Challenger - Trains,8651 UP Challenger 4-6-6-4, #3954, Black & Graphite, Coal Tender, No-Sound / DCC-Ready, N