It has many advantages for heavy high-speed service, namely, large and well-proportioned boiler, practically unlimited grate area, fire-box of favourable proportions for firing, fairly low centre of gravity, short coupling-rods, and, finally, a combination of the safe and smooth riding qualities of the fourcoupled bogie type, with great steaming capacity and moderate axle loads.
In still rarer cases both a leading and a trailing bogie have been fitted.
Per hour the resistance of the engine and tender is 33 lb per ton, and the resistance of a train of bogie coaches about 14 lb per ton.
Thus 4-4-2 represents a bogie engine with four-coupled wheels and one pair of trailing wheels, the wellknown Atlantic type; 4-2-2 represents a bogie engine with a single pair of driving-wheels and a pair of trailing wheels; 0-4-4 represents an engine with four-coupled wheels and a trailing bogie, and 4-4-o an engine with four-coupled wheels and a leading bogie.
Occasionally a somewhat similar type is designed with the bogie under the fire-box and a single leading axle forward under the smoke-box - an arrangement in favour for suburban tank engines.
It soon led to an increase in the length of the vehicles; thus in 1885 the Midland railway had four-wheeled bogie third-class carriages, with bodies 43 ft.
Long, holding seventy persons in seven compartments and weighing nearly 18 tons, and sixwheeled bogie composite carriages, 54 ft.
For instance, fourwheeled bogie third-class corridor carriages employed on the Midland railway at the beginning of the 10th century weighed nearly 25 tons, and had bodies measuring 50 ft.; yet they held only 36 passengers, because not only had the number of compartments been reduced to six, as compared with seven in the somewhat shorter carriage of 1885, by the introduction of a lavatory at each end, but each compartment held only 6 persons, instead of 10, owing to the narrowing of its width by the corridor.