Why do railways in Britain always refer to the direction of travel as "up" or "down"? Why do you hear phrases like, "the 10:25 up train to London" or "the down loop"? In the US, they use the train's direction to describe the track, e.g. "eastbound" or "westbound" but many other railways like Australia, India, China and Japan use "up" and down", after the British system. But why?
It's simply a remnant of history. Railways were first introduced in the 18th century to help get coal from mines. Wagons, with their wheels running on specially laid tracks, were pulled by men or horses from the mine to the shipping point. The bulk of coal was sent by sea because roads were so bad.
Tracks were laid from mines to docks. Normally, the mines were inland and higher than sea level, so the coal was transported down to sea level where the docks were. Thus, the direction away from the mine became known as "down" and the return trip as "up". Now, "down" is the track leading away from the main terminus (usually London), while the "up" track is towards London.
There are anomalies. The District Railway, now London Underground's District Line, had its original terminus at Mansion House. Its services ran west from there, so the line to Ealing, Richmond or Wimbledon was the "down" line and the eastward track became the "up" line. When the route was extended east to Whitechapel and Barking, the track running east from Mansion House became the "down" line, and the return track the "up" line. Thus the name of the track changed as the train passed through Mansion House. When the system was electrified in 1905, using American money and technology, they adopted the American "eastbound" and "westbound" and these remain with us to this day.