Double-headed, compound, articulated Mallets. Here's a head-on view of Columbia River Belt Line #7 "Skookum" and Clover Valley Lumber Company #4 charging westbound with a mixed freight in the late afternoon sun, just outside Sunol, California. To my knowledge, as of February, 2020, the Niles Canyon Railway, operated by the Pacific Locomotive Association, is the only railroad in the country capable of doing this. At some point in the near future, it is expected that the Black Hills Central Railroad in South Dakota will also have this capability. They have a pair of 2-6-6-2T saddle-tankers, similar to the Clover Valley Locomotive seen here. One of the Black Hills Central's engines is operational and the other is in the final stages of restoration.