In railway parlance the “up” line is the one leading to London and the “down” away. To complicate matters railway people will talk of up and down lines even for routes that don’t go to London.
The only explanation for the convention is this. I don’t now how true it is.
*In the early days of railways there were few trains and the timetable could be written on a single sheet, with the most important terminus at the top, and stations going down the left. Trains in both directions were written on this. So if London was at the top then if you wanted trains from London you read DOWN the page, and trains to London were read UP the page.
When there were two tracks the track corresponding to the up direction of the timetable was the Up line and the other the Down line.
If one end of the line covered was London trains to it were always UP (hence you always say “I’m going up to London”). Otherwise the main centre of the railway company might be used, eg Derby in the case of the Midland.
On all lines there was always an assumed Up and Down direction for ease of referring to the tracks (if more than one) and the trains. This terminology is still used today.*