... are attached to the underframe of the carriage at each end by a hefty post. These bolsters ride on volute springs. Two on each side became standard to help with higher speeds as the early ones had stability problems over 55 mph. So eight in all. These springs not only carry the weight of the vehicle but also dealt with the turning and lateral displacement of the bogie truck. They are connected to the bogie frame by hanger bolts that transmit the loads to brackets mounted on the bogie solebars. The hanger bolts have spherical bearings at each end. Effectively the carriage body becomes the bolster. He had two distinct suspension designs. Inner and outer. The outer ones are referred to as centreless but that refers to the lack of bolster, not guide pin. (As just pointed out by a_nummelin jn his post). The outer units had restrictions with longer bogies as they interfered with the amount of pivot so fell out of favour with the development of longer coaches and matching longer bogies. Hope this is of use. Matt M. posted ...