Going back to the reason for the original post the OP seemed to suggest either 4 tracking or grade separation. That conjured up images of a double deck 4 track railway!
Would grade separating junctions really produce the same capacity improvement as 4 tracking?
There are already 4 tracks at Diddyland.
Grade separation simply allows you to get from one line to another without using the line(s) inbetween.
So going to Didcot as an example. A service travelling from London to Oxford on the "fast down".
On Google Maps, you'll see that there is a bridge to the East of Tesco. At this point the London train has to move from the "fast down" onto the "fast up" and travel along it until it reaches Tesco.
It then moves from the "fast down" to the "relief up"
It then continues on the "relief up" until it reaches Aldi, then it moves over to the Didcot avoiding "down" line.
So for the time that it undertakes that move, the only other movement that can happen on the signalling gap between Didcot Station and the bridge East of Tesco is a service on the "Up Avoiding" line to the "relief up" line.