Steam locos designations. What do all those numbers mean?

Steam Locos Designations Guide


Ever wondered what all those numbers mean? Why is it a 4-6-0? Why did its number change and what is a Pacific? Here’s a simple guide to steam locomotive designations.

Tank and Tender Locomotives

Tank locomotives carry the water they need in tanks attached to the locomotive.

  • Side tanks (T) have two tanks, one on either side of the boiler which reach down to the footplate (the level the crew are standing on).
  • Pannier tanks (PT) have two tanks in a similar arrangement but there is a gap between the bottom of the tank and the footplate.
  • Saddle tanks (ST) have a large tank wrapped over the top of the boiler like a saddle,
  • Well tanks (W) have a tank mounted underneath the locomotive but this arrangement is usually only used on small locomotives.

Coal is carried in a bunker which was usually behind the cab.

These locomotives were used for shunting or shorter trips with frequent stops so they did not need to carry large amounts of water and coal.

Class N2 0-6-2T tank engine 1744, built at the North British Locomotive works in 1921. Ex BR number 69523, LNER locomotive number 4744,
GWR 16xx class 0-6-0PT 1638 at Sheringham. Built at Swindon in 1951 the loco is preserved by the Kent & East Sussex Railway in Swindon green GWR livery.
0-6-0 ST Tank Locomotove 1700 Wissington built in 1938 for use on the British Sugar Corporations line at Wissington. Pictured at Sheringham station.

Tender locomotives carry both coal and water in a tender pulled behind the locomotive. When steam locomotives were the dominant form of motive power on British railways the tender could sometimes be refilled with water without stopping from a trough between the rails using a scoop but water crane mounted at the end of the station platform was more often used.

9F 2-10-0 92203 Black Prince at Sheringham station on the North Norfolk Railway. This heavy goods loco was built in 1953 is preserved in BR black livery and is owned by David Shepherd.

More photos of 9F 2-10-0 Black Prince

Wheel Arrangements

Steam locos are classed by their wheel arrangements. The first number is the smaller wheels on the leading bogie or pony truck, the second is the number of large driving wheels linked together by connecting rods and the third the trailing wheels.

Witherslack Hall below is a 4-6-0 Four small wheels – six driving wheels – no trailing wheels

Hall class 4-6-0 6990 Witherslack Hall on the Great Central Railway Loughborough

Wheel arrangements were originally given names as this helped allocate locomotives to specific tasks. The suitability of a locomotive to a specific load or route often depended as much on its wheel arrangement as to its power.

As the variations of wheel arrangements increased it became impractical to use names and the Whyte Notation system using figures for the wheel arrangement was introduced.

The original names for some of the more common wheel arrangements continued to be used. Some still in use today are:

Pacific. A 4-6-2 locomotive. Usually, these are the large express locomotives, Flying Scotsman and Mallard being the most famous examples but Pacific could also be applied to a tank engine with the 4-6-2 wheel arrangement.

Atlantic. A 4-4-2 locomotive. Again this could also be applied to either a tender or tank engine.

Mogul. A 2-6-0.


When the railway system was founded in the UK each line was run by a separate company. Each company built or commissioned its own locos and ran them on track of different gauges (the distance between the inside edges of the two rails).

In 1846 the Gauge Act mandated that all new lines should be a standard gauge of 4ft 81/2in (1435mm). This gauge was adopted by many railways worldwide.

Locomotive Numbers & Classes

As the railway system grew many of the companies merged but by the beginning of the twentieth century there were still lots of separate companies, many of which were not economic. In 1923 they were grouped into the big four; the LMS (London Midland and Scottish), LNER (London and North Eastern Railway), GWR (Great Western Railway) and the (SR) Southern Railway.

When the big four were created they renamed the types of locomotives they inherited from the smaller companies and renumbered them. This was not always an immediate process. The GWR renumbered some locos in 1923 and then again in 1946. The LNER renumbered their locos in 1946, whilst the LMS renumbered them in 1923 and again in 1932 (the J27 loco below is a good example).

In 1948 the big four were nationalized and became British Railways.

The new British Railways retained the locomotive class designations but the locomotives were renumbered. This involved adding a 3 for SR, 4 for LMS, and 6 for LNER locos to the beginning of the existing number. GWR locomotives retained their original numbers (see 6990 Witherslack Hall above which was an ex GWR locomotive).

For example. The J27 in these photographs was built as a NER (North Eastern Railway) Class P2 with the number 2392. The LNER renamed all 0-6-0 locomotives it now owned Jxx, so the P2 became a J27.

The LNER then renumbered it 5894 and it became 65894 on British Railways.

Photo of J27 Locomotive 65894 (NER P3 2392) approaching Goathland station on North York Moors Railway
J27 Locomotive 65894 (NER P3 2392) approaching Goathland station on North York Moors Railway

It is not necessary to know all this but it does make clear why some locomotives carried different classifications and numbers during their lives. It also makes captioning your railway photographs and searching for the history of a loco easier.


Technically a steam engine is a device that provides power just the same as a petrol engine. When it is mounted on wheels it becomes a steam locomotive. Really a steam locomotive should not be called a steam engine any more than a car should be called a petrol engine but we all use the term.

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