Historically the Great Western Railway, connecting London with southwest England and Wales.
MAIN LINE: Paddington-Reading and beyond
4 tracks Paddington-Reading
From south to north, pairs of tracks identified as Main, Relief.
Airport Terminal 5
Airport Terminal 4 (1 track)
Windsor (1 track/diesel)
Marlow (1 track/diesel)
Henley (1 track/diesel)
All lines 2 tracks unless noted. All lines AC electrification unless noted.
Number Of Platforms/Tracks: 14
Platforms numbered south to north
Platform 1-10 - Main Line
Platform 6-7 - Main Line/Heathrow Express
Platform 11-14 - Relief Line
Whereas most four track lines from London have their pairs of tracks identified as Fast or Slow, the tracks on this line are identified as Main and Relief, respectively.
Wikipedia link containing further station information and history.
Zoomable Google Maps along the various lines.