The Whyte notation for classifying steam locomotives by wheel arrangement was devised by Frederick Methvan Whyte and came into use in the early twentieth century encouraged by an editorial in American Engineer and Railroad Journal (December 1900). Whyte's system counts the number of leading wheels, then the number of driving wheels, and finally the number of trailing wheels, groups of numbers being separated by dashes. Other classification schemes, like UIC classification and the French, Turkish and Swiss locomotive and railcar classification systems for steam locomotives, count axles rather than wheels.
Thus, a locomotive like Gordon with two leading axles (and thus four wheels) in front, then three driving axles (six wheels) and followed by one trailing axle (two wheels) would classified as a 4-6-2.
Thomas is a locomotive with three driving axles, who has side tanks. This makes him an 0-6-0T
Edward is a tender locomotive with two leading axles, and two driving axles. This makes him an 4-4-0.
The suffix T indicates a tank locomotive; otherwise, a tender locomotive is assumed. In British practice, this is sometimes extended to indicate the type of tank locomotive: T means side tank, PT pannier tank, ST saddle tank, WT well tank. T+T means a tank locomotive that has a tender for additional coal or water capacity.
In Europe, the suffix R could mean rack (0-6-0RT) or it could mean reversible (0-6-0TR). The latter case being the Bi-cabine locomotives used in France.
The suffix F indicates a fireless locomotive (0-4-0F). Note that this locomotive has no tender.
Other suffixes have been used at times, including ng for Narrow gauge railway locomotives (i.e., less than 56.5 in / 1435 mm) and CA or ca for compressed air (i.e., running on compressed air from a tank instead of steam).