Most new vehicles are equipped with roller bearing axles as shown here. An advantage in roller bearing axles is less frequent maintenance and adaptability to hydraulic brakes.
These come in sizes from 1” to 2½” as measured at the square portion of the axle and are often referred to as ‘standard track’ and ‘wide track’. The standard track will measure about 48” between spindles and the wide track will measure close to 54”. Overall lengths are available up to 75” by special order.
1” and 1 1/8” are common to most light vehicles such as doctor’s buggies and phaetons. The surreys, spring wagons and Vis-à-vis will have the 1¼” sizes. Vehicles which accommodate more passengers, such as wagonettes, ought to be up to 1½” size. When you go into the light coaches the 1¾” axle is appropriate, while the larger coaches, drays and freighters will require the larger 2” to 2½” axles.
Roller Bearing Axles can be viewed here.