CAUTION! While I have done my best to be accurate,  I cannot guarantee that this page is error free.  
If you know of corrects, please do contact me via the link above right.

 Locomotive Wheel Arrangement Classifications (all types of motive power)

Characters  Meaning
A, B, etc.Number of powered axles within a single frame (chassis or bogie) where A=1, B=2, etc.
CoIndicates individually powered axles. Absence indicates coupled axles.
1, 2, etc.Number of free running axles within a single frame (chassis or bogie or pony truck).
2'Pivoted frame indicator. (Bogie or pony truck.)
(1'C)Running and driving axles in a swivelling frame (i.e. not chassis). E.G. in articulated locomotives.
+Dividing point between wheel groups of articulate locos or complex bogies.
Examples  2'C1' = ooOOOo (4-6-2) :: Co'Co' = OOO–OOO :: (2'C)C1' = (ooOOO)OOOo (4-6+6-2 Mallett)

 Secondary letters for Steam Locomotives and Tenders

Characters  Meaning
h
n
Heißdampf – superheated steam.
Nassdampf (saturated steam)
2, 3, 4Number of cylinders
vVerbundmaschinen – Compound (high/low pressure) locomotive.
tTenderlokomotiv – tank engine.
Examples  h3 = superheated 3 cylinder :: h4v = superheated 4 cylinder compound :: n2 = saturated steam 2 cylinder

 Classification of Tenders

Characters  Meaning
Axles As for locomotives above.
TSchleppentender indication – Tender indicator.
numberWater capacity in m3.
K or Kon*Kondenstender – Condensing tender.
Examples 3T13 = 3-axle carrying 13 m3 of water :: 5T16Ko* = Condensing tender with 5 axles carrying 16m3.
*This appears to be the correct format. However, I have seen several different ways of indicating this tender, e.g. K3'2'T16.

 Secondary letters for Electric Locomotives

Characters  Meaning
Character 1 = Current Supply
g
w
d
Gleichstrom – Direct Current
Wechselstrom – Alternating Current ( single phase )
Drehstrom (Dreiphasenwechselstrom) – Alternating Current ( three phase )
Character 2 = Motors.
1, 2, etc.Number of Motors
Character(s) 3(+4) = Drive Type
k

u

e

t
gf
 
Kurbelantrieb ohne Vorgelege – Coupling rod drive without an intermediate gear / intermediate transmission / primary reduction gearing
Kurbelantrieb mit Vorgelege (übersetzt) – Coupling rod drive with an intermediate gear / intermediate transmission / primary reduction gearing.
Einzelachsantrieb mit im Rahmen gelagerten Motoren – Individual axle drives with high-mounted motors (i.e. up in the body)
Tatzlagerantrieb – Individual drive on each axle with nose-suspended or axle-hung motor
Einzalachsantrieb mit im Drehgestell gelagerten Motoren mit gefedertem Antrieb – Bogie mounted motors with sprung drive to the wheel sets
Examples
 
BAY EP3/6/DRG E36: 1'C2' w1k = 1 in-body AC motor and coupling rods (OO-OO) •
E44/BR144: Bo'Bo' w4t = 4 AC axle mounted motors  •  E170: Bo'Bo' g4t = 4 DC axle mounted motors

 Secondary letters for Diesel Locomotives

Characters  Meaning
Character 1 = Fuel Type
d
b
Diesel
Benzin – petrol (US: gas)
Character 2 = Transmission
m
e
h
p
Mechanisch – Mechanical
Elektrisch – Electric
Hydraulisch – Hydraulic
Pneumatisch – Pneumatic (only applied to V120 001 / V3201).
Examples
 
DRG V16/V140: 1'C1' dh = Diesel hydraulic with coupled drivers  •  DB V200: B'B' dh = Diesel hydraulic  •
DRG V3201/V120: 2'C2' dp = Diesel pneumatic with coupled drivers.

 Railcar/Multiple Unit Wheel Arrangement Classifications

Generally, the system used for locomotives also applies to railcars and multiple units except that, if the latter are in fixed formations, in which case all the units are included.
Examples Meaning
Examples Class ET87. 3-car set: (2)1+B'1 w2k+1(2), (oo)o+(OO)o+o(oo) - AC, 2 motors, coupled drive
VT62: B'2'

 Secondary letters for Railcars

Characters  Meaning
   
Examples