METHODS OF HAULING IN TUNNELS 57 



through without delay. To what extent this may be done, or 

 whether it can be done at all or not, depends upon the method 

 of excavation and other local conditions. In soft-ground 

 tunnels excavated by the English or Austrian methods, 

 it U quite possible to lay the permanent tracks at first, since 

 the whole section is excavated at once, and the excavation is 

 kept but a little ahead of the completed tunnel. In rock 

 tunnels, where the heading is driven far ahead of the com- 

 pleted section, it is, of course, impossible to keep the perma- 

 nent track close to the advance work, and narrow-gauge tracks 

 must be laid in the heading. The same thing is true in soft- 

 ground tunnels driven by successive headings and drifts. In 

 these cases, therefore, 

 where narrow-gauge 

 tracks have to be used 

 for some portions of 

 the work anyway, the 

 question comes up 

 whether it is preferable 



Fio. 37 Box-Car for Tunnel Work. 



to use temporary 



narrow-gauge tracks throughout, or to lay the permanent track 

 as far ahead as possible, and then extend narrow-gauge tracks 

 t the advance excavation. In the latter case it will, of course, 

 be necessary to trans-ship each load from the narrow-gauge to 

 the standard-gauge cars, or vice versa, which means extra cost 

 and trouble. To avoid this, the method is sometimes adopted 

 of laying a third mil. between the standard-gauge rails, so that 

 either standard- or narrow-gauge cars may be transported over 

 the line. Whatever form the local conditions may require the 

 system of construction tracks to assume, it may lie set down as 

 a general rule that the permanent tracks should !K> kept as far 

 advanced as fxissihle, and temporary tracks employed only 

 where the permanent tracks are impracticable. 



The motive power employed for hauling in tunnels may be 

 furnishc 1 l>y animals or by mechanical motors. Animal power 



