Every name of locomotive tells someone what wheels are under it. In some cases, two or three names fit the same wheels. A "T" signifies a tank-engine type, with no tender. Much of the world counts wheels, but France and Spain (among others) counts axles. The 0 is pronounced "oh," and not "zero." A Mogul is thus called "twosixoh."
OO Four-wheel switcher 0-4-0
OOO Six-wheel switcher 0-6-0
OOOO Eight-wheel switcher 0-8-0
OOOOO Ten-wheel switcher 0-10-0
OOOOOo Union switcher (from the Union Railroad, near Chicago.) 0-10-2
oOO Four-coupled 2-4-0
oOOo Columbia 2-4-2
oOOooT Forney, (could also be a Mason Bogie) 2-4-4T
oOOO Mogul (first ones sent to Japan) 2-6-0
oOOOo Prairie 2-6-2
oOOOO Consolidation (first ones sent to the newly-formed Lehigh Valley Railroad) 2-8-0
oOOOOo Mikado (first ones sent to Japan) 2-8-2
oOOOOoo Berkshire (first ones built for the hills of Mass and Penn.) 2-8-4
oOOOOO Decapod (literally, "ten feet.") 2-10-0
oOOOOOo Santa Fe 2-10-2
oOOOOOoo Texas (called Selkirks in Canada) 2-10-4
ooOO American (or "For-for-oh") 4-4-0
ooOOo Atlantic (first ones built for the Atlantic Coast Line) 4-4-2
ooOOoo (Canadian ones called Jubilee, for the 50th anniversary Canadian Pacific.) 4-4-4
ooOOO Ten-wheeler 4-6-0
ooOOOo Pacific (first ones built for New Zealand) 4-6-2
ooOOOoo Hudson (Hudson River Route of New York Central) 4-6-4
ooOOOooT Baltic (used as commuter engines) 4-6-4T
ooOOOO Twelve-wheeler 4-8-0
ooOOOOo Mountain 4-8-2
ooOOOOoo Northern (also Niagara, also Mohawk, also Greenbrier) 4-8-4
ooOOOOO Mastadon 4-10-0
ooOOOOOo Southern Pacific 4-10-2
ooOOOOOOo Union Pacific 4-12-2
ooOOOOOOOoo It has no name, but they were used in the former Soviet Union, and I just thought the idea of seven coupled axles had to be included here somewhere. 4-14-4
All the above are what was called a "rigid frame engine," with no flexibility between the drive wheels. You then got into Articulated locomotives, which had two separate engines mounted under one boiler. The engines could pivot independently of one another, allowing the locomotive to go around curves. Some of these also had names:
oOOO OOO ooo Allegheny (Chesapeake & Ohio) 2-6-6-6
ooOOO OOOoo Challenger (Union Pacific, Rio Grande) 4-6-6-4
oOOOO OOOOoo Yellowstone (Duluth, Missabe & Iron Range) 2-8-8-4
ooOOOO OOOOoo Big Boy (Union Pacific) 4-8-8-4
oOOOO OOOO OOOOo Triplex (Erie) 2-8-8-8-2. Rear engine actually under the tender.
To add complication to all this, each railway had its "Class" of engines. A Canadian Pacific G-5, for example, was a light, modern 4-6-2 Pacific. On the Pennsylvania, the same G-5 class was a heavy 4-6-0 Ten wheeler.
