When a train enters a track section, the wheels and axles place a shunt (short) on the
track circuit. This creates a low resistance current path from one rail to the other and
in parallel with the existing ballast resistance and relay coil. When maximum current
from the battery is reached because of current flow through the relay coils, ballast
resistance and low resistance path created by the train shunt, the relay armature drops.
Most of the current flows through the low resistance shunt path. This reduces the
current in the relay sufficiently to cause the armature to drop, thereby opening the
front contacts. In Figure 7-15, the heavy dark arrows indicate the high current path
through the shunt.
Figure