In Whyte notation, 2-4-4-2 refers to a railroad steam locomotive that has two leading wheels followed by two sets of four coupled driving wheels and two trailing wheels.
Other equivalent classifications are:
1BB1 (also known as German classification and Italian classification)
120+021
23+23
2/3+2/3
The UIC classification is refined to (1'B)B1' for a Mallet locomotive.
A similar wheel arrangement has been used for Garratt locomotives, but it is referred to as 2-4-0+0-4-2 since both engine units can pivot.
This articulated wheel arrangement was rare in North America; example was the Mallet locomotive. Most were built as logging locomotives, presumably to better negotiate the uneven (and often temporary) trackwork that characterized such operations. The added mechanical complexity was found to be of limited value, as reflected in their modest production and use.[1] There is one known surviving example: Columbia River Belt Line No. 7 "Skookum" (former Little River No. 126), built by Baldwin Locomotive Works in June 1909. It was retired and abandoned in place in the forest following a derailment in 1955. As of January 2023, it has been restored to operating condition following a 15 year rebuild at the Oregon Coast Scenic Railroad shop in Garibaldi, Oregon. The engine currently resides at the Niles Canyon Railway in Sunol California.[2] [3] [4] [5]
Built by ALCO (shop # 53970), this locomotive is a Mallet Compound engine, built for the Taupo Totara Timber Company for use on the 52miles TTT Railway between Putāruru and Mokai in the North Island. It is now preserved on the Glenbrook Vintage Railway, near Auckland, New Zealand and now carries the number 4. The engine is currently out of service awaiting overhaul, but can still be seen at the railway's Pukeoware workshops.