sketch below depicts a commuter train operating at some cruise speed vC
behind another train. To maintain safe separations between trains,
the wayside control system detects the preceding train and transmits speed
commands to the "train in trail." The actual speed of the preceding
train is not known, only the location of its rear-most car.
Headway, the time between successive trains passing a given point,
is an important determinant of railroad performance (see Streetcar