Trackside Prints

Locomotive wheel arrangements, decoded

Type a steam engine's Whyte notation, like 4-6-2, or a diesel or electric's UIC code, like Co'Co'. You get the wheels and axles spelled out, and the old type name where there is one.

Decode it

4-6-2: Pacific or St. Paul

  • 4 leading wheels, 6 driving wheels, 2 trailing wheels

12 wheels in all, on 6 axles, 3 of them driving. Colvin's 1906 table lists a 4-6-2 as Pacific or St. Paul.

Co'Co': 6 axles

  • Co': 3 driving axles, each with its own motor, on a bogie
  • Co': 3 driving axles, each with its own motor, on a bogie

6 axles are driven out of 6.

The 15 type names in Colvin's 1906 table
WhyteName, as printed
4-2-2Single Driver
4-4-0American
4-4-2Atlantic
4-6-0Ten Wheel
4-6-2Pacific or St. Paul
4-8-0Twelve Wheel
4-10-0Mastodon
2-4-2Columbia
2-6-0Mogul
2-6-2Prairie
2-8-0Consolidation
2-8-2Mikado or Calumet
2-10-0Decapod
2-10-2Santa Fe
2-12-0Centipede
A black pannier tank engine numbered 1501 standing at Wansford station under a grey winter sky
1501 at Wansford, on the Nene Valley Railway. Photo: kitmasterbloke, CC BY 2.0

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How to read Whyte notation

Whyte notation counts wheels, front to back, with dashes between the groups: leading wheels, driving wheels, then trailing wheels. A 4-6-2 has four wheels at the front, six driving wheels and two at the back (Whyte notation). An engine with two sets of driving wheels gets four numbers, like the 4-8-8-4 Big Boy (UIC classification). A Garratt, two engine units joined by one boiler, gets a plus sign between them.

Letters on the end mark a tank engine: T for tank, ST for saddle tank, WT for well tank and PT for pannier tank, as the Whyte notation page lists them. No letters means an engine with a tender.

A green steam locomotive numbered 60103 with a long tender, standing on a main line under overhead wires
60103 Flying Scotsman, a 4-6-2: count the wheels from the front. Photo: HawkeyeUK, CC BY-SA 2.0

The type names

Many arrangements picked up names. The tool uses the fifteen in F. H. Colvin's Railroad Pocket-Book of 1906, page L-9, printed as he wrote them: Atlantic for 4-4-2, "Pacific or St. Paul" for 4-6-2, Mogul for 2-6-0, Prairie for 2-6-2 and the rest. We show only those, so we never guess a name.

How to read a UIC code

The UIC system counts axles, not wheels. A capital letter is a group of driving axles, A for one, B for two, C for three. A number is axles with no drive. A small o means each axle has its own motor, and a prime mark means the group is on a bogie. So Co'Co' is two bogies, each with three motored axles (UIC classification).

Sources: Wikipedia, "Whyte notation" and "UIC classification of locomotive axle arrangements" (CC BY-SA 4.0); F. H. Colvin, The Railroad Pocket-Book (1906), p. L-9. Read 30 October 2026.

Written by Craig Fearn, Trackside Prints. Last updated .

Questions, answered

What does 4-6-2 mean?
Four leading wheels, six driving wheels and two trailing wheels. Colvin's 1906 table lists it as "Pacific or St. Paul".
What is the difference between Bo-Bo and Co-Co?
Bo-Bo is two bogies of two motored axles each, four in all. Co-Co is two bogies of three, six in all.
What does the PT in 0-6-0PT mean?
Pannier tank. Wikipedia's Whyte notation page lists PT for a pannier tank engine; no letters would mean an engine with a tender.