1 MARCH 2000
5. TRACK STRUCTURE.
a. Design Objective. Among its various functions, the track system serves to distribute the large,
concentrated wheel loads longitudinally, laterally, and vertically away from the wheel contact area on the
rail surface. A well designed, constructed, and maintained track will distribute the loads in a relatively
uniform fashion, with each component supporting its share of the load. The role of the designer is to
select a track structure (or changes to an existing track) so that the intended wheel loads will be properly
supported, without overstressing any of the four main track system components: rail, ties, ballast, and
b. Load Distribution. As figure 6-10 shows, when a wheel is centered over a tie, the tie directly beneath
the load will generally carry less than half of that wheel load, with the remainder supported by two ties on
either side. Beneath the wheel, the pressures are distributed (reduced) approximately as shown in figure
6-11. With a wheel-rail contact area of about sq in., stresses are reduced from 60,000 psi at the top of
the rail to about 10 psi at the depth of the subgrade surface. While the actual load and pressure
distributions will vary with wheel load, track design, and track condition, the two figures illustrate a realistic
case for good track.
Figure 6-10. Example Load Distribution along the Track