Double heading

In railroad terminology, double heading indicates the use of two locomotives at the front of a train, each operated individually by its own crew. The practice of triple-heading involves the use of three locomotives.

Double heading is most common with steam locomotives, but is also practised with diesel locomotives. It is not strictly the same practice as two or more diesel or electric locomotives working 'in multiple' (or 'multiple-working'), where both (or all) locomotives are controlled by a single driver in the cab of the leading locomotive.

Double heading is practised for a number of reasons:

Double heading requires careful cooperation between the engine crews, and is a skilled technique, otherwise one locomotive's wheels could slip, which could stall the train or even cause a derailment.

The risks of double heading as well as its costs (fuel and maintenance costs for the engines, wages for their crews) have led railroads to seek alternative solutions. Electrification has been used in many cases. The Milwaukee Road was able to switch from triple-headed steam locomotives to a single electric locomotive. The costs of running extra steam locomotives were eliminated, and average train speeds increased because it was no longer necessary to attach and detach the locomotives. In Britain, the Midland Railway used to use double-heading often, because it built only small, light locomotives, which were often not powerful enough to haul the trains alone. Several accidents on the Midland system were indirectly caused by this 'small engine policy' and the resulting reliance on double-heading. Some were caused by trains stalling despite being double-headed (such as the 1913 Ais Gill rail accident), while others were caused by excessive light-engine movements as locomotives that had been used for double-heading returned to their depots (the Hawes Junction rail crash in 1910). When the Midland was absorbed into the London, Midland and Scottish Railway, this practice was stopped because it was uneconomical, and more powerful locomotives were built.

...
Wikipedia