It would double the number of trains between Falmer and Brighton, where additional capcity is most needed.
I think you are overstating the problems caused by the trains going the other way serving different destinations. People currently travelling to HH via Lewes would switch to going via...
It's due to be resignalled. Difficult to find specific details on what is happening, but this is from the Staff and Visitor Travel Plan:
"In the medium term future Network Rail are planning to up-grade the signalling on the Brighton to Lewes section of the east coast line, which will double the...