The maximum speed is usually used for the actual cant setting, although a lower speed may be used taking into account the running safety of low-speed trains and maintenance. In this case or when the cant setting for maximum speed exceeds the maximum cant, a cant deficiency 1/20 of the gauge is permitted.
Cant diminishing can be divided into linear diminishing used by cubic parabolas and curve diminishing employed by sine transition curves.
3.4 Improvement of Turnouts
The properties of straight sides and turnout sides differ significantly in the straight turnouts that are normally used. Turnouts easily produce large vibrations and lateral force due to their structure which is more complex than that of general tracks. Straight sides have the following characteristics.
a As crossings have gaps, these areas become worn and produce strong vertical vibrations due to the impact of wheels.
b Wheel flanges impact strongly with crossing guard rails and wing rails, producing substantial lateral force.
c There are a large number of joints, including weak insulation joints and the loose joints of switch heels.
d Gauge widening not required by the straight sides takes place to secure turnout slack, and as the gradient is steep, unstable wheel movement easily occurs.
Compared to general curves, turnouts have a number of shortcomings including the following.
a There is not cant nor transition curves.
b Points have entry angles.
c Not enough slack is produced.
As a result, speed restrictions through turnouts are stricter than those for general sections.
Current high-speed turnouts which allow running along straight sides at 130 km/h have kept the above deficiencies to a minimum. In particular, flexible switches, reinforced guard rails, welding of joints in turnouts and other improvements have been made to remove the running speed restriction on straight sides.
Replacing to turnout of higher crossing number is the key factor for increasing speeds on the turnout side, but this may prove difficult due to wiring problems. Furthermore, high costs have led to the running speed on the straight side being prioritized. Recently, a #20 symmetrical turnout with a lead radius of 2,151 m was laid using a curve crossing, allowing a high speed of 120 km/h for both sides.
