Helper
An additional locomotive added to a train to assist over a steep grade — usually cut in at the rear or middle, and cut out at the summit.
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A helper is an additional locomotive (or set of locomotives) coupled to a train to assist it over a section of railway too steep for the road power alone to climb. Helpers are typically cut into the train at the foot of a known grade, push or pull alongside the road locomotives over the climb, and are cut off at or just past the summit — sometimes only minutes after being attached. The practice is as old as steam railroading and survives intact in modern diesel operation because some grades simply cannot be flattened.
There are two configurations. A pusher (the common form on long, heavy freight) attaches to the rear of the train and pushes uphill, with the rearmost car taking the buff load on its couplers. A swing helper or mid-train helper is inserted into the middle of the consist and controlled remotely by the lead engineer through DPU radio links — this distributes the additional pull along the train and keeps couplers under tension rather than compression. On heritage steam lines, double-heading (two engines coupled together at the front) is the most photogenic helper form, but it has been almost entirely replaced operationally by rear-end and mid-train pushers.
Famous helper districts worldwide include the Saluda Grade in North Carolina (closed since 2001, but for decades the steepest mainline grade in the USA at 4.7%), the Norfolk Southern Allegheny crossing, the Helper, Utah area on the former Rio Grande, Tehachapi Loop in California, the St Gotthard summit in Switzerland (now bypassed by the base tunnel), and the Cajon Pass east of Los Angeles. Each generated specific operating practices, distinctive locomotive assignments, and a body of railfan lore.
For railfans, the helper move itself is photographable in three distinct phases: the cut-in, the climb with extra horn and exhaust on every car, and the cut-out at the summit. The cut-out, in particular, makes for one of the more dramatic operational moments on a modern railway — the helper drops off behind a still-moving train and reverses back down to wait for the next eastbound.