As against this construction of an elaborate network of strategical lines towards and along _the Russian frontier_, there must be put the fact that although, by this means, Germany acquired the power to effect a great and speedy concentration of troops on the frontier itself, her locomotives and rolling stock would not be able to cross into Russia and run on the railways there because of _the difference in gauge_. On the eastern frontier the question as to how an invasion in large force could be effected was, consequently, quite different from that which would present itself on the western frontiers, where the railway gauges of Belgium, Luxemburg and France were the same as those of Germany. It was certain that whenever, in the event of war, German troops were able to enter Russian territory, Russia would withdraw into the interior or else destroy such of her locomotives and rolling stock as the enemy might otherwise utilise for his own purpose. If, therefore, the Germans wanted to use the existing Russian lines, they would either have to build, in advance, locomotives and rolling stock capable of running thereon, or they would have to convert the Russian gauge of 5 feet to the German gauge of 4 feet 8½ inches, so that German trains could run on the other side of the frontier. As already remarked on page 61, the reduction of the broader gauge into a narrower one would involve fewer engineering difficulties than an expansion of the German gauge into the Russian gauge; yet even the former procedure, if carried out over any considerable length of line, would take up a good deal of time, and this would be still more the case if the Russians, when they retreated, destroyed the railway track and bridges behind them, as they might confidently be expected to do. Dependence, again, on the existing lines across the frontier would, apart from questions of conversion and reconstruction, still give Germany only a very small number of railway routes into Russia, and these, also, at points where the opposition offered might be especially active.