These two graphs relate to each other. Check them out.
These are the amounts of ties used for railroads. More ties means more track. The consumption went down in the 1920's - 1940's because of the world wars. Lots of unused track was taken up because of the need for metal. Click for larger image. Forest History
This is the amount of track laid. There was exponential growth around the time air brakes came around because people felt safer on trains. Click for larger image. Forest History