They're opening it in three stages. PAD - RDG is open SHN - LIV is open PAD - ABW will open as the first "core section" The two branch sections will join later in the year when they work out some more tech issues between the various safety systems ie the core to Abbey Wood uses one system, the GWR and GEML use two different systems
https://forums.dovetailgames.com/th...-to-paddington-at-reading-and-heathrow.51985/ Here you have more details of that suggestion
The Underground sections will be Stop Signal Overrun fest the US NTSB term for UK RAIB ( Rail accident investigation board) Train Sim World 2 SPAD Signal Passed at Danger.