Strategical railways, whether designed for defensive or aggressive purposes, may, in turn, be divided into two main groups, (1) those that constitute a network of lines; and (2) single or individual lines for short or long distances. A network of strategical railways is generally found in direct association with frontiers. Single or individual strategical lines fall into various groups including (1) short lines or branches running out to some point on or near to a frontier; (2) single lines carried for long distances, and, possibly, crossing entire continents; (3) circular or short lines, connecting different railway systems with one another, in order to facilitate the movement of troops during mobilisation or concentration or for defensive purposes in the event of invasion; (4) lines passing round cities or large towns in order to avoid delay of troop trains; and (5) lines for coast defence. The ideal conditions for a network of strategical railways was already a subject of discussion in Germany in 1842, when Pönitz brought forward his proposal that that country should provide herself with such a system. There were, he said, theorists who designed, on paper, strategical railways which, starting from a common centre, radiated in straight lines to different points on the frontier and were connected with one another by parallel or intersecting lines of railway on the principle of a geometrical design, or, he might have added, of a spider's web. Pönitz admitted the excellence of the idea, suggesting that if there were, indeed, a group of lines to the frontier connected by cross lines allowing of a complete interchange of traffic, the enemy would never know at what point a sudden advance in force might not be made, while the linking up of the entire system would greatly facilitate working. In practice, however, as he proceeded to point out, this ideal system could not be fully adopted, partly because the planning of railways is influenced by the configuration of the country, which may not permit of geometrical designs for iron roads; and partly because the trunk lines of national systems of rail communication had already been laid by private enterprise on the principle of catering for the social and economic needs of the community and of returning interest on capital expenditure, rather than of serving military or political purposes.