articulated locomotives

No one really wants the complication of articulated locomotives! By far the easiest engines to maintain are those which are rigid: meaning that the driving wheels remain parallel to the boiler at all times.  As traffic increases on a railway, the company is faced with new problems. More trains can be operated with existing equipment but this will be limited to the signalling capability of the line and the number of locomotives available. In addition, more trains mean more staff who have to be paid and managed.

The alternative is to run longer trains and this means more powerful locomotives are needed. Here one will very quickly reach a limit for a number of reasons.

• The size of boilers are limited by the loading gauge of the line.

• Track and civil engineering structures all have a maximum axle loading so the heavier locomotive will have to spread the weight over more wheels.

• More driving wheels limit the radius of curves that the locomotive will negotiate.

This is when everybody and their dog gets out paper and start to design their own articulated locomotive. In fact some designs have been so bizarre that they must have been designed by the dog! Here we shall restrict our copy to common solutions found on narrow gauge lines.

The first narrow gauge railway to need articulated locomotives was the venerable Festiniog Railway. The tiny 0-4-0 tender locos built by England were quickly inadequate for much of the traffic and a new design by Robert Fairlie was built called 'Little Wonder'. He had supplied double Fairlie locomotives elsewhere which had been hugely unsuccessful but the new Festiniog engine became a resounding success.

The design consisted of two pivoting power bogies mounted under a double boiler. This removed firebox size restrictions but the fireman was presented with twice the work. As with all articulated locomotives, one weakness has always been maintaining the integrity of the flexible steam joints but Fairlie did manage to solve many of the problems.

It is true to say that most double Fairlie locomotives were unsuccessful and only the Festiniog continue to use the configuration.

Single Fairlies were also built and ran on a number of lines.

'Taliesin' an 0-4-4 single Fairlie built new by the Festiniog Railway

Péchot-Bourdon locomotives were built in France for military use and are nothing more than a developed Fairlie. Over 70,  0-4-0 0-4-0 locomotives were built and operated with mixed success. Larger examples were also built.

The original design by Jean Meyer of France became the most common of articulated locomotives despite suffering from a problem of not being able to include large fire boxes. Kitson solved the problem by moving the bogies apart. These engines were exported around the world and became larger and larger in size.

'Monarch', which moulders at the Welshpool and Llanfair Railway was originally built for Bowaters Railway Kent. This is an original Meyer design built by Bagnall. A circular marine firebox was fitted which did not contribute to its success and 'Monarch' has become the locomotive that no one wants to operate.

The Swiss, Anatole Mallet, developed his design as a flexible engine. The rear power truck forms part of the rigid structure and the front bogie hinges from the rear. A true Mallet is a compound. That is, the high pressure steam exhausted from the rear cylinders is transferred to the larger cylinders of the front bogie and used for a second time. The advantage of the Mallet is that the flexible joints are kept to a minimum and those that are used have minimal movement.

The disadvantage is that the front of the boiler overhangs while negotiating curves which reduces stability and restricts the locomotive's speed.

Mallets were always very popular in Europe but never really caught on in the UK.

The United States developed the Mallet into behemoths of staggering proportions. One design even involved a flexible joint in the boiler (designed by a dog).

Big Boy

The Garratt is the most famous of all the articulated locomotives and its advantages are well known. However it does have one very bad characteristic: its length... This leads to very long steam pipes and other things having to be stretched to vast distances. There are only two known examples of compounded Garratts, these being K1 and the Burma Railways loco.

the World's first Garratt - K1 now at the Welsh Highland Railway

newly rebuilt Garratt No 87 at Boston Lodge, Festiniog Railway

Some really huge Garratts have been built and the Garratt is certainly one of the most successful articulated designs. Needless to say, there are a bunch of Garratt look-alikes which appeared in order to circumnavigate the patent.

A number of designs were also developed which allow the addition of an extra driving wheel which possessed some sideways movement. One such system can be found on the NG15 Mikado (2-8-2). In this case, the Krauss-Helmholtz bogie.

The requirements of logging railways have spawned a whole range of unique articulated designs. Locomotives were required to run on terrible track, sometimes, even, just on logs, climb very stiff grades and be very serviceable. Speed was certainly never an issue.

Shay locomotive

Shay locomotives had regular fire-tube boilers offset to the left to provide space for a two or three cylinder "motor," mounted vertically on the right with longitudinal drive shafts extending fore and aft from the crankshaft at wheel axle height. These shafts had universal joints and square sliding slip joints to accommodate motion of the swivelling trucks. Each axle was driven by a separate bevel gear, and used no side rods.

Driving all wheels, including those of the tender, together with small diameter wheels were the strength of these engines, their entire weight developing tractive effort. A high ratio of piston strokes to wheel revolutions allowed them to run at partial slip, where a conventional rod engine would spin its drive wheels and burn rails, losing all traction.

Shay locomotives were often known as sidewinders for their side-mounted drive shafts. Most were built for use in the United States, while many found their way to 30 additional countries, territories or provinces.

Although the Shay was the most common geared locomotive, it had a significant flaw that was not corrected. Because the drive shaft lies outside the trucks, instead of along the centreline, truck rotation when following track curvature causes substantial drive line length change, unlike the central drive shafts of Heisler locomotives and Climax locomotives. In modern drive shafts, this effect is accommodated by roller splines instead of bronze slip joints (shown between "Sonora's universal joints") that lose their ability to slide under high torque.

Wreck photographs show Shay locomotives, before or after uphill curves, where they failed to respond to change in track curvature, thereby running off the track "for no apparent reason." Some texts refer to these locomotives as "rail spreaders" and "flange hounds," both characteristics of trucks that do not steer freely under heavy load.

Other disadvantages include the noise of the gearing, and the very low top speed.

The Climax locomotive has two steam cylinders were attached to a transmission located under the centre of the locomotive frame. This transmission drives drive shafts running forward and rearward to gearboxes in each driving truck.

Unlike the somewhat similar Heisler design, there were no side rods on the trucks. The gearboxes drove both axles on each truck.

Many loggers considered the Climax superior to the Shay in hauling capability and stability, particularly in a smaller locomotive, although the ride was characteristically rough for the crew.

The Heisler locomotive was the last variant of the three major types of geared steam locomotive, Charles L. Heisler receiving a patent for the design in 1892 following the construction of a prototype in 1891. Somewhat similar to a Climax locomotive, Heisler's design featured two cylinders canted inwards at a 45 degree angle to form a 'vee-twin' arrangement. Power then went to a longitudinal driveshaft that drove the outboard axle on each powered truck. The inboard axle on each truck was then driven from the outboard one by external side (coupling) rods. The Heisler was the fastest of the geared steam locomotive designs, and yet was still claimed by its manufacturer to have the same low speed hauling ability.