The purpose of sleepers is to keep the track gauge fixed and distribute the stress transmitted from the rails directly to the ground. In modern types of track system, which do not allow for sliding, sleepers also have the task of blocking thermal expansion of the rails and sliding due to braking or starting.

In order for the carriage to adapt to the curve to be travelled, the fifth wheel must be able to move sideways, and this is achieved by placing it on a support suspended by four tie rods from the carriage frame itself. This support is called a swinging crossbar.

In three-axle bogies with a wheelbase of 3.70 m there are two swinging crossbeams, symmetrical to the intermediate axle and connected by a longitudinal beam carrying the fifth wheel. According to the design, the bogies are made from two gull-wing shaped side members made from folded and welded steel plates, joined by welding by two crossbeams.