This Thameslink line goes from three bridges to Brighton the route is only 28 and 32 minutes running on class 700 EMUS on thaneslink.
Class 700's only operate on the Arun Valley line between Three Bridges and Horsham, also the Arun Valley doesn't run direct to Brighton, the Brighton mainline does and that route is already in Tsw2.
You can actually extend this towards Portsmouth Southampton Littlehampton via Bognor Regis. This is seen on BML
