This concentration of traffic units is largely due to technical improvements of various kinds. Foremost among these has been the development of the steel rail. In 1880 more than seven-tenths of our mileage was still equipped with iron rails. Rapid progress ensued during the next ten years, upward of eighty per cent. being in steel rails by 1890. At the present time, the proportion is above ninety-eight per cent. In fact, no iron rails have been made for many years, except for repairs and on insignificant branch lines in remote parts of the country. A steady increase in the weight of the rails has ensued. The standard rail for main lines until the Civil War weighed fifty-six pounds to the yard. In the seventies this was increased to sixty-three and above; in the latter eighties the best practice was to use seventy-five pound sections. Since 1900, they frequently run as high as one hundred pounds, in regions of dense traffic. Few main lines of track now average less than seventy-five pounds. It is this increase in the use and size of steel rails which has permitted improvements in rolling stock. But, on the other hand, grave dissatisfaction with the quality of the rails manufactured of late years, particularly since the establishment of practical monopoly under the United States Steel Corporation has become manifest. Numerous accidents due to breakage of rails, especially since 1905, have revealed either defects in manufacture or an undue load imposed by heavier rolling stock, too high speed, or both. The matter has become steadily worse. In 1902 the Interstate Commerce Commission ascribed seventy-eight accidents to broken rails. Nine years later the number had risen to 249. The need of improvement is now fully recognized on all sides.