Charles Benjamin Collett OBE JP MInstCE MIMechE[1] (10 September 1871 – 5 April 1952) was Chief Mechanical Engineer of the Great Western Railway from 1922 to 1941.

By the time Churchward retired, his 4-cylinder Star class locomotives were becoming inadequate for the increasing loads and speeds expected of express passenger trains.

Instead, Collett enlarged the Star design, leaving the wheels the same but with bigger cylinders and a new standard boiler that would not exceed the 19+1⁄2 ton permitted axle load.

Both engines acquitted themselves well, but the performance of the smaller Castle led Gresley to investigate the cause and redesign the A1's valves, as well as give them a higher boiler pressure.

This at least had the effect of drawing the attention of General Manager Sir Felix Pole to the restrictive permitted axle loads which had constrained the usefulness of this prestige symbol.

When larger locomotives were needed, Sir Felix instructed the Civil Engineer to ensure that the main lines (Paddington–Plymouth, via Westbury or Bristol, and Paddington–Wolverhampton via Bicester) could carry a 22+1⁄2 ton axle load.

The very first one, No 6000 King George V, was shipped off to America where it led the parade of engines at the Baltimore and Ohio Railroad Centenary Exhibition, and whence it returned carrying a commemorative bell on its front buffer beam.

[14] Stanier paid it the compliment of using its principal dimensions (Boiler pressure, cylinder size and wheel diameter—and therefore tractive effort) in his first design, the 4-6-2 LMS Princess Royal Class.

The story goes that Collett took a paperweight model of a King, and smeared plasticine over it to produce an outline for the drawing office to work from.

As The Great Western Railway Magazine put it, "Rigorous application of the principles of scientific streamlining becomes not only difficult but practically inexpedient, as the net reduction in the total resistance may be relatively small.

[20] Both of these engines, and no others, were given a full-page photograph in the GWR's 1935 publication Swindon Works and its place in Great Western Railway History, but without any further descriptive text.

Although the GWR retained its identity by virtue of being grouped with many much smaller railways, it did make Collett responsible overnight for over 800 locomotives, of a wide variety of designs, many in a state of disrepair.

2 boiler, and as many other standard parts as possible, the cylinder castings, wheels, hornblocks, valve gear and three-bar motion crosshead were all new designs.

Investigation showed that in the transfer of the standard Stephenson valve-gear arrangement from outside-cylinder use to an inside-cylinder design, the function of the rocker shaft (a substantial bearing that transferred the valve motion through the frames to drive the steam chest) in withstanding off-axis forces was not appreciated, and inadequate support was provided for the end of the valve spindle.

[27] The Running Department asked him to build an enlarged version of this engine, as crews had a tendency to expect too much of it (possibly due to its similarity with larger classes).

Further batches totalling 259 locos were built up till 1943, and—together with his successor Hawksworth's 71 Modified Halls—made up the largest class of named locomotives on the GWR.

A further batch of 20 was built in 1930, but due to the depression of the late 1920s, the short-haul South Wales coal trains for which they were intended had decreased, and they were placed in storage.

For this he developed an existing design, the 2721 class of 0-6-0ST/PT, fitting them with a Belpaire boiler pressed to 200 lbf/in2 (1.4 MPa), a larger 3 ton 6 cwt bunker, an all-over cab and improved valve events.

All of these were built by outside suppliers: W. G. Bagnall (50), Kerr, Stuart (25), North British (50), Yorkshire Engine Co. (25), Armstrong Whitworth (25), and Beyer, Peacock (25).

For other duties, Collett created a variant with a higher boiler pressure of 180 lbf/in2 (1.2 MPa), but not auto-fitted, and 50 of these were produced numbered in the 7400 series.

Very similar in frames and motion to the 5700 Pannier tanks, but with cylinders designed to support a No 10 standard boiler, they were given a Yellow route classification (slightly heavier than the Dean Goods).

These delightful engines, being small and nowhere near the end of their working life when steam was withdrawn on British Railways, were popular targets for saving, and four have been preserved.

This economical combination of the better parts of older engines was produced as the Earl class, though they were widely termed the Dukedogs due to their origins.

Collett was aware that certain members of the GWR Board of Directors (whom he considered pompous) desired that their names should be given to suitable steam engines, and he therefore applied them to Earl class locomotives.

[52] Minor developments, of less significance to the GWR, were a brief experiment with a Sentinel geared locomotive,[53] and the building/rebuilding of three 2-6-2T engines for the narrow-gauge Vale of Rheidol Railway.

In 1934 he introduced the use of Zeiss optical apparatus during locomotive manufacturing to improve the accuracy with which frames and bearing, cylinders and motion were lined up.

[59] A gifted technical engineer, who could look at existing designs and reliably improve them, Collett produced a standardized fleet of locomotives ideally suited to the GWR's requirements.

In 1924 he reported (in a paper to the World Power Conference) Castle class coal consumption of 2.83 lb per drawbar-horsepower hour, a figure dismissed as too good to be true by many engineers, but taken seriously by Gresley after the locomotive exchange of 1925.

[60] Collett has received criticism by contemporary engineers and later railway historians for undertaking very little innovation in his designs, instead sticking with Churchward's style in every case.

Arguably this meant that by the time Collett retired the superiority of Great Western locomotives was lost to more modern designs, particularly those of William Stanier, who worked at Swindon before moving to the LMS in 1932.