| 1868 |
The line extends from ‣ to ‣ |
| 1892 |
Line extensions reaches Aylesbury |
| 1904 |
The ‣ branch opens |
| 1905 |
The first electric trains appear and are gradually introduced across the whole line, apart from the line beyond ‣ |
| 1925 |
The ‣ branch opens |
| 1932 |
Another branch, to Stanmore opens, but this becomes part of the ‣ line in 1939 |
| 1961 |
The steam trains operating north of ‣ stop as the line is electrified to ‣ and ‣. Services beyond ‣ are taken over by British Rail (now Chiltern Railways) |
| 2012 |
A new fleet of electric trains are introduced, the first on the Underground to feature air conditioning and full-length, walk-through interiors |