In offering the second edition it may be stated that the bulk of the revision work was required because it was necessary to conform to the changes made a few years ago by the Interstate Commerce Commission in the classification of operating expenses, and this necessitated a practical rewriting of the chapters on Operating Expenses, Distances, Curvature, and Grade.

The changes in the early chapters consisted principally of enlarging tables and diagrams and bringing them up to date.

In many cases, instead of merely substituting other data for that compiled about six years ago, the author thought it would be advisable, in order to make forcible conclusions, to give both sets of figures and point out the changes occurring in the last five or six years and their significance. He also added some tabular statements on various kinds of traffic, for instance, VIa, VI6, and VIc, the data for which were not available when the first edition was published; and he believes that its value, presented as it is, in condensed figures, will be seen, and the average reader will thereby avoid the necessity of compiling these matters for himself.

The added tabular forms, in the chapter on Operating Expenses, on wages of employees, point out not only the almost uniform increase in wages in that direction, but also the variations at any one time in different sections of the country, so that a person estimating wages in one section would not be misled by the scale of wages actually paid in another.

In addition to the foregoing, some blemishes which had been discovered in the first edition were eradicated in those portions which were rewritten, and others, which only required minor typographical changes, were corrected.

It is believed that these changes and corrections will bring the work up to date and establish it as a standard.