Down (railway terminology)

Railroad directions are used to describe train directions on rail systems. The terms used may be derived from such sources as compass directions, altitude directions, or other directions. However, the railroad directions frequently vary from the actual directions, so that, for example, a "northbound" train may really be headed west over some segments of its trip, or a train going "down" may actually be increasing its elevation. Railroad directions are often specific to system, country, or region.

Many rail systems use the concept of a center (usually a major city) to define rail directions.

In British practice, railway directions are usually described as up and down, with up being towards a major location. This convention is applied not only to the trains and the tracks, but also to items of lineside equipment and to areas near a track. Since British trains run on the left, the "up" side of a line is on the left when proceeding in the "up" direction. The names originate from the early railways, where trains would run up the hills to the mines, and down to the ports.

On most of the network, "up" is the direction towards London. In most of Scotland, with the exception of the West and East Coast Main Lines, "up" is towards Edinburgh. The Valley Lines network around Cardiff has its own peculiar usage, relating to the original meaning of traveling "up" and "down" the valley. On the former Midland Railway "up" was towards Derby. On the Northern Ireland Railways network, "up" generally means toward Belfast (the specific "zero" milepost varying from line to line); except for cross-border services to Dublin, where Belfast is "down". Mileposts normally increase in the "down" direction, but there are exceptions, such as the Trowbridge line between Bathampton Junction and Hawkeridge Junction, where mileage increases in the "up" direction.

...
Wikipedia