I made this wiki entry to address those hyphenated number tags you'll see in some train themed posts. Some examples of these tags would be 4-4-0_american, 2-8-2_mikado, and 4-6-0.

Whyte notation system for classifying steam locomotives by front-to-rear wheel configuration.
(<# of pilot wheels> - <# of driver wheels> - <# of trailing wheels>)

_OO________ '0-4-0' "Switcher"
_OOO_______ '0-6-0' "Switcher"
_OOOO______ '0-8-0' "Switcher"
_OOOOO_____ '0-10-0' "Switcher"
_OOOOOO____ '0-12-0' "Twelve-coupled"

_oO_________ '2-2-0' "Planet"
_oOo________ '2-2-2' "Single"
_oOoo_______ '2-2-4'
_ooOo_______ '4-2-2' "Bicycle"
_ooOoo______ '4-2-4 "Huntington"
_oOO________ '2-4-0'
_oOOo_______ '2-4-2' "Columbia"
_oOOoo______ '2-4-4' "Forney/Mason Bogie"
_oOOO_______ '2-6-0' "Mogul"
_oOOOo______ '2-6-2' "Prairie"
_oOOOoo_____ '2-6-4' "Adriatic"
_oOOOO______ '2-8-0' "Consolidation"
_oOOOOo_____ '2-8-2' "Mikado/MacArthur"
_oOOOOoo____ '2-8-4' "Berkshire/Kanawah"
_oOOOOO_____ '2-10-0' "Decapod"
_oOOOOOo____ '2-10-2' "Santa Fe"
_oOOOOOoo___ '2-10-4' "Texas/Colorado/Selkirk"
_oOOOOOO____ '2-12-0' "Centipede"
_oOOOOOOo___ '2-12-2'
_oOOOOOOoo__ '2-12-4'

_ooOO________ '4-4-0' "American"
_ooOOo_______ '4-4-2 "Atlantic"
_ooOOoo______ '4-4-4' "Reading"
_ooOOO_______ '4-6-0' "Ten Wheeler"
_ooOOOo______ '4-6-2' "Pacific"
_ooOOOoo_____ '4-6-4' "Baltic/Hudson"
_ooOOOO______ '4-8-0 "Mastodon"
_ooOOOOo_____ '4-8-2' "Mountian/Mohawk"
_ooOOOOoo____ '4-8-4' "Northern/Niagra"
_ooOOOOO_____ '4-10-0' "Mastodon"
_ooOOOOOo____ '4-10-2' "Southern Pacific"
_ooOOOOOO____ '4-12-0' (Probably doesn't exist)
_ooOOOOOOo___ '4-12-2' "Union Pacific"

On articulated giants, drivers are counted on their respective frames. I will not continue to list them, but the whyte classification for the world's largest steam locomotive is '4-8-8-4' "Big Boy/Yellowstone"