A shunt causes the signals to go red, restricting train movement.
Got it.
a shunt is used to mirror the
conditions which the track can see when a train is passing
through a powered section. This tells the dispatch office
that something is on the tracks within that section. It is
irrelevant whether that something is a train, equipment,
or workers as dispatch isn’t able to tell the difference.
However, what the shunt does is inform them that
something is on the tracks at that location. This allows
them to divert trains from entering the area, reducing
the danger to equipment and personnel, and preventing
interference with whatever operations may be ongoing.