1. Extension beyond Reading to Oxford. Quite a few services in services made start or end at Oxford. Obviously these "end" at Reading with NPc drivers taking the game further. Also, where to the single track lines from Twyford (and another, can't remember) go to? A previous version of Train Simulator had the full LP to Oxford route. 2. Sandbox or more freedom in Services. Ability to create your own service and route plan, what train you will need for it (eg 3x C166 or extra long C43).
In Answer to your question of the Single Track Lines from Twyford go to Wargrave, Shiplake and Henley on Thames. From Reading you got Reading West, Mortimer, Bramley and Basingstoke aswell as Tilehurst and Pagbourne that goes on the Reading to Oxford Route and Also from Reading you have Winnersh, Wokingham, Crowthorne, Sandhurst, Blackwater, Farnborough North, North Camp, Ash, Etc