Historically the London & North Western Railway, evolving to the West Coast Main Line connecting London with Scotland.
MAIN LINE: Euston-Milton Keynes
4 tracks Euston-Milton Keynes
From east to west, pairs of tracks identified as Slow, Fast.
Separate 2 track DC Electric Line parallels to east, Camden Junction to Wembley, and parallels to west, Wembley to Watford Junction. DC Electric Line also used by London Underground Bakerloo Line north of Queens Park.
St. Albans (Watford Junction-St. Albans Abbey/1 track)
All lines 2 tracks unless noted. All lines AC electrification unless noted, including DC Electric Line DC electrification.
Number Of Platforms/Tracks: 18
Platforms numbered east to west
Platform 1-7 - Fast Line via Track "X"
Platform 8-11 - Slow Line
Platform 12-18 - Fast Line
North of Euston Station, the four tracks are Slow to the east and Fast to the west. Immediately north of the station, Track "X" passes underneath the Slow tracks, enabling certain Fast Line trains to use the lowest numbered platforms to the east.
Further north of the station, the DC Electric Line diverges from the Slow Line.
Wikipedia link containing further station information and history.
Zoomable Google Maps along the various lines.