The Rail data frame has 18 rows and 2 columns.
This data frame contains the following columns:
an ordered factor identifying the rail on which the measurement was made.
a numeric vector giving the travel time for ultrasonic head-waves in the rail (nanoseconds). The value given is the original travel time minus 36,100 nanoseconds.
Devore (2000, Example 10.10, p. 427) cites data from an article in Materials Evaluation on ``a study of travel time for a certain type of wave that results from longitudinal stress of rails used for railroad track.''