Another problem with running additional services on the Portsmouth Lines route is the lack of workarounds if one line gets blocked by signalling problems, failed trains and so on. Crossovers are very limited and for most of the route there is nothing remotely like bidirectional signalling. Dorking has a couple of platforms for looping and terminations, if the timetable allows it, which is surprisingly rare; there is a crossover with limited functionality at Leatherhead, there are a couple of opportunities to be resourceful with platforms at Epsom, and then further north, where it's even busier, you're really quite limited in what you can do. The next significant opportunities to turn trains round are really only at West Croydon (if you're being inventive with the turnback), Balham, Streatham or, if you head onto the Wallingtons, perhaps Selhurst (for the depot only).
The semaphore signalling between Dorking and Horsham was Tyers one-wire two position with intermediate SBs at Holmwood and Warnham. With Holmwood switched out (only opened in the peaks for the starters or terminators), the section was Lodge Farm IBS to Warnham on the down, or about 20 minutes with a stopping train. The present set-up (if Traksy berths are correct) appears to replicate the old capacity when Holmwood was switched in.
Nearly replicates it, certainly. However, there are quirks, such as the crossover at Holmwood no longer being in existence. This makes working around faults and failures quite difficult, but fortunately the line beyond Dorking is quiet enough to prevent this becoming too much of an issue.
Interestingly, the Dorking-Horsham section of the Portsmouth Lines has a small amount of jointed track, and track circuits are not used in some places. Axle counters are instead used from (approximately) country side of Betchworth Tunnel, until you get just north of Warnham.
Despite a very large amount of electrical infrastructure for such a comparatively short and less-well-used bit of line, the third rail is not especially reliable, indeed I was delayed on several services during one recent morning peak because of this. The positioning of the signal sections is also a constraint; as could be implied from what you've written, the line can handle about 3tph reliably in each direction, including occasional non-stop runs between Dorking and Horsham. So long as you don't try more than that, the next biggest constraint can often be the throat at the north of Horsham itself. Trains from Dorking fairly regularly get held there for around 3 to 10 extra minutes, which is not attractive for trying to make connections.
