Understanding railways is vital to understanding warfare in WWI, and to a lesser extent in WWII. Trains represent the most efficient way of moving troops, supplies and munitions from point A to point B (with the exception of ships, but they can't go inland). A train is quicker than marching troops or horse-drawn wagons, and uses less fuel than a column of trucks. During WWI, the vast majority of supplies would have travelled by train to reach the front. Supplies and troops would be moved from ports, barracks or factories by rail to a forward 'railhead', where they would be unloaded for further shipment to the front.

One of the key factors for describing a railway network is the 'gauge' - the distance between the rails. The gauge of a railway defines how big the trains that travel on it can be. A broader gauge means that the wagons are broader, and hence can be larger. Similarly, the locomotives are broader, and hence are more powerful. Broader gauge trains are generally more stable, allowing higher speeds. The broader gauge also spreads the weight of the train over a larger area, meaning it can be more heavily laden. Narrow gauges can use tighter curves; this means they can avoid difficult terrain features, simplifying the engineering of the route. Trains designed for use on rails of one gauge cannot easily be used on rails of another - at least, not without significant engineering taking place. Most of the European participants in WWI had standardised on a single gauge - 'Stevenson' gauge or 'Standard' gauge - with a wheelbase of 4ft, 8.5in. Russia, meanwhile, opted for a 'Broad' gauge of 5 ft. 'Narrow' gauge railways, with gauges of about 2ft, were used by Germany, Britain, France, the USA and the Russian Empire to move supplies from the railheads closer to the front. As the Russians used a different gauge from Austria-Hungary and Germany, it made it difficult for either to invade the other. Trains from one gauge could not be used on the other. This meant that as Russian forces advanced into Germany (or vice versa), the trains carrying the supplies and reinforcements they needed had to stop at the border; there, they had to be offloaded and shipped forwards using less efficient means. Captured rolling stock could be used - but this relied on actually capturing that rolling stock, which was by no means guaranteed. Standard gauge railways could be converted to broad gauge, but this was manpower-intensive, and generally proceeded slower than the troops could advance. This problem would slow the Russian advance into Germany in 1914 and 1944-5, and the German advance in the opposite direction in 1941.

The issue with the British and French railways, meanwhile, was an issue of capacity. A railway line can only carry so many trains - to give the simplest example, only a given number will physically fit onto the line, and this goes down when you add gaps between them for safety considerations. There are also only so many trains available within the railway system, as railways do not have an infinite supply of locomotives and rolling stock. The British Expeditionary Force deployed to the Continent in August 1914 consisted of six infantry divisions and five cavalry brigades. The plan for deploying this force had been worked out pre-war by General Henry Wilson, in collaboration with the French. It required significant amounts of staff work because it needed to take place at the same time as the French were deploying their own army. The French mobilisation would take up a significant amount of the railway capacity available. The British deployment had to work with the minimal spare capacity available; this meant that they had to work to specific pre-planned timetables, with little flexibility. Changing them would have meant freeing up lines or rolling stock that were being used by the French forces that were operating near the British. Doing so would have required changing the plans for French forces further afield to compensate for these changes. In other words, changing the plan for the British force would have required changing the plan for the French deployment at the same time.