Wheel Arrangement
There are various ways to describe a locomotives wheel arrangement, the most common used for steam locomotives is the Whyte System which consists of three digits, the first specifying the number of wheels before the driving wheels, then the driving wheels, followed by any trailing wheels. This may also be followed by a T to indicate a side tank engine, ST to indicate a saddle tank, PT for pannier tank or WT for well tank.
The same system can be used to describe internal combustion locomotives without bogies. In these cases it is usual to append letters to describe the power and transmission method. e.g. 0-6-0PM would be a petrol engine with mechanical transmission, DH would be diesel hydraulic, DE would be diesel electric.
Another typical method for diesels without bogies simply states the number of wheels (it assumes that all wheels are driven) followed by the power and transmission method as above. e.g. 4wDM is a four wheel diesel with mechanical transmission.
The Continental System is usually used for diesels with bogies. Powered axles are indicated by letters and non-powered by numbers (ie. A=1 powered axle, B=2 etc), if the axles are independently powered, a small letter o is added after the main letter. Thus a locomotive with two bogies, each having two powered axles will be Bo-Bo if the axles are independently powered (the norm for diesel electrics where each axle has its own electric motor), or B-B if the axles are not independently powered (which is the norm for diesel hydraulics). An exception to the usual rule is when there is an un-powered axle between the powered ones (eg class 31). This arrangement is described as A1A-A1A.

The home of model making.