It is, in the first place, especially necessary to bear in mind that the railway, as a means of transport, must needs be regarded from a point of view wholly different from that which would apply to ordinary roads. On the latter any sort of vehicle can be used, and there are, generally, alternative roads along which traffic can pass, in case of need. Railroads are not only available exclusively for vehicles constructed to run upon them, but the degree of their usefulness is limited by such considerations as the number of separate routes to a given destination; the important matters of detail as to whether the lines are single track or double track and whether they are on the level or have heavy gradients; the number of locomotives and the amount of rolling stock available; the extent of the station and siding accommodation; the provision or non-provision of adequate facilities for loading and unloading; and, in war time, the damage or destruction of a particular line or lines by the enemy. The amount of traffic it is possible to convey between certain points in a given time may thus be wholly controlled by the physical conditions of the railway concerned, and such conditions may be incapable of modification by the railway staffs, in case of a sudden emergency, however great their desire to do everything that is in their power. In the next place, all these physical conditions may vary on different railway systems, and even on different sections of the same system. It does not, therefore, necessarily follow that military requirements which can be complied with on one line or in one district can be responded to as readily, if at all, under another and totally different set of conditions elsewhere; though it is conceivable that a military commander or officer who fails to realise this fact may, if he is left to deal direct with the railway people, become very angry indeed at non-compliance with his demands, and resent protests that what he asks for cannot be done at one place although it may have been done at another.