Tank locomotive

This type originated about 1840 and quickly became popular for industrial tasks, and later for shunting and shorter-distance main line duties.

However, the more common form of side tank date from the 1840s; one of the first of these was supplied by George England and Co. of New Cross to the contractors building the Seaford branch line for the London Brighton and South Coast Railway in 1848.

The length of side tanks was often limited in order to give access to the valve gear (inside motion).

Side tanks almost all stopped at, or before, the end of the boiler barrel, with the smokebox protruding ahead.

It gave a greater water supply, but limited the size of the boiler and restricted access to it for cleaning.

A few American locomotives used saddle tanks that only covered the boiler barrel, forward of the firebox.

Locomotives were built for the Belgian State and for la Société Générale d'Exploitatation (SGE), a private company grouping smaller secondary lines.

[10] In the United Kingdom, pannier tank locomotives were used almost exclusively by the Great Western Railway.

[13] This does not restrict access to the boiler, but space is limited there, and the design is therefore not suitable for locomotives that need a good usable range before refilling.

A rear tank is an essential component of the American Forney type of locomotive, which is a 4-4-0 American-type with wheels reversed.

[18] This rare design was used for the same reasons as the wing tank but provided slightly greater water capacity.

[21] This may have been to increase the water capacity, to equalise the weight distribution, or else improve the stability by lowering the centre of gravity.

There are two main positions for bunkers on tank locomotives: to the rear of the cab (as illustrated in the left of the images below), a position typically used on locomotives with a trailing carrying axle or a trailing bogie; or on top of and to one side of the firebox, a positioning typically used in cases where the firebox overhangs the rear driving axle, as this counterbalances the overhanging weight of the firebox, stabilising the locomotive.

One of the major advantages of the Garratt form of articulation is the maintenance of the locomotive's centre-of-gravity over or inside the track centre-line when rounding curves.

Contractor's locomotives were usually saddle or well tank types (see above) but required several adaptations to make them suitable for their task.

[6] At the same time, they had to be very powerful with good traction as they would often have to haul trains of wagons up very steep gradients, such as the sides of railway embankments or spoil heaps.

Many were designed so that large iron ballast blocks could be fitted to the frames when extra weight and traction was required, then removed when it was not.

To handle long trains of loose-coupled (and often un-sprung) wagons, contractor's locomotives usually had very effective steam-powered brakes.

Some tram engines were fitted with a roof and enclosed sides, giving them an appearance more like a goods wagon than a locomotive.

With their limited fuel and water capacity, they were not favoured in areas where long runs between stops were the norm.

In the United Kingdom, they were frequently used for shunting and piloting duties, suburban passenger services and local freight.