71 



weight of the standard rail has been increased ; but the foundation of the 

 entire structure of rail transportation is still the wooden tie. 



Faced with the possibility of ultimate exhaustion of the supply of 

 wooden ties through depletion of the forest resources, railroad engineers 

 have for three decades experimented with substitutes in order to be ready 

 for the transition when wood was no longer available ; but even in the 

 face of threatened serious shortage of supply and in spite of rapidly 

 increasing costs, cross-ties continue to be made almost exclusively from 

 wood. 



Not only is the number of ties other than wood now in use infinitesi- 

 mal, but this substitution is not yet increasing at a rate indicating a ten- 

 dency to abandon wood. For the year 1920, Census figures show that 

 out of a total of 86,829,307 ties laid in Class I track for replacements 

 in the U. S., but 154,378 were of material other than wood, or 18/100 of 

 1 per cent. 



The two factors which will determine the rate of possible substi- 

 tution for wood in this field are service and cost. Service means the 

 durability or preservation of the tie in a serviceable condition capable 

 of supporting the rail and preserving the gage of the track. Ties cease 

 to render service when they break, decay, get out of shape, crumble, or 

 in any way lose their form and structure. 



Ties in modern road-beds must withstand terrific shock from traffic. 

 Resiliency rather than rigidity is required, and wood presents this quality, 

 while more rigid metal or cement lacks it. Cement ties crumble rapidly 

 and are no longer considered of value. The rigid steel tie, in spite of 

 its less desirable qualities might ultimately supplant wood, but this sub- 

 stitution dei)ends also on relative costs and demonstrated period of serv- 

 ice. These factors still favor the wooden tie. 



Wooden ties fail from two sets of causes, namely, decay, and me- 

 chanical .stress which causes the tie to wear out. For these reasons cross- 

 ties were originally made only from woods which possessed the maxi- 

 mum resistance to shock, combined with the greatest resistance to decay 

 when exposed to the surface conditions in the track. White oak and 

 chestnut were the favorite hardwoods, and heart pine the most widely 

 used material in coniferous regions. 



TIMBER PRESERVATION 



During the past decade the supply of the so-called class "U", or un- 

 treated ties, those naturally resistant to decay and therefore not requir- 



