What is the difference between an up and down line?

Generally speaking, an up line travels towards a major town or city and a down line away from it, on main lines in England this is usually London while in Scotland it's Edinburgh (except for Scottish mainlines to London which remain in favour of the English capital).

Of course, there are exceptions to this rule as not all lines originate in London. Branch lines, for example, will designate the line in the direction of the dominant town or junction on the route as the up and the line coming away from it as the down while cross country lines with several cities of roughly the same size along the route will change from down to up as the route nears the next big city.

Some mainlines too were not in favour of the London look with the Midlands route between London and Derby considering the direction of travel towards Derby as the up and away from Derby as the down so the rule must be taken with a pinch of salt.

Up and down lines should also not be confused with up and down trains where the train's base is used as a point of reference rather than the importance or size of towns passed along the way.