As a general ride, it is considered preferable to lay the permanent tracks at once, and do all the hauling upon them, so that as soon as the tunnel is completed, trains may pass 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 is 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 completed section, it is, of course, impossible to keep the permanent 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 to use temporary narrow-gauge tracks throughout, or to lay the permanent track as far ahead as possible, and then extend narrow-gauge tracks to 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 rail 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 be set down as a general rule that the permanent tracks should be kept as far advanced as possible, and temporary tracks employed only where the permanent tracks are impracticable. The motive power employed for hauling in tunnels may be furnished by animals or by mechanical motors. Animal power is generally employed in short tunnels and in the advance headings and galleries. In long tunnels, or where the excavated material has to be transported some distance away from the tunnel, mechanical power is preferable, for obvious reasons. The motors most used are small steam locomotives, special compressed-air locomotives, and electric motors. Compressed air and electric locomotives are built in various forms, and are particularly well adapted for tunnel work because of their small dimensions, and freedom from smoke and heat.