New York City Subway chaining is a method to precisely specify locations along the New York City Subway lines. It measures distances from a fixed point, called
chaining zero, following the twists and turns of the railroad line, so that the distance described is understood to be the "railroad distance," not the distance by the most direct route ("
as the crow flies"). It is an example of the
chainage system, instead of the
milepost or mileage system.