East Croydon-Gatwick Airport is surely a contender for having a high volume of delays and blocks of single/multiple lines taken - delays themselves simply stack up due to the sheer number of trains in the area, other than during the very early hours of some mornings. And I'm not just saying that because I know those lines well. The actual frequency of incidents requiring remedial work just seems as though it is quite high on the scale of these things.
Things are improving - Stoats Nest Jn is much more reliable after teething troubles were sorted after renewals, fatalities have reduced somewhat due to station fences (being discussed elsewhere), and hopefully landslip risks will significantly reduce as and when the weather improves; also, station staff are becoming increasingly resilient & experienced when something does go wrong. But there are still many occurrences of congestion & knock-on delays from elsewhere - including the Southern MML; routes via Tonbridge, Watford Jn & Southampton; you name it! So the last few years have really shown the scope of problems spiralling out of control.
With diversion routes remaining questionable/met by scepticism (BML2) or simply lengthy & rarely-used (Dorking), I cannot see any solution other than to continue with incremental improvements - some of which I can foresee coming if the new TSGN franchise can deliver better recovery strategies in London, some extra track/platforms on the routes (Redhill, London Bridge), and possibly better timetabling at junctions, all of which should help.
This is not to mention the disruption which can be caused by engineering works on either line between Purley & Earlswood for the purposes of renewals and improvements. Hopefully much can continue to be learnt from the likes of the Gatwick/Stoats Nest Jn Christmas Blockades.
Also on the Southern network, the line between Barnham & Bognor/Southampton/Portsmouth always appears to have extremely frequent requirements for trains to start/terminate short, owing to congestion in various places. Because the Southampton & Portsmouth routes are shared in part by SWT, the situation is improving, as the SWT/NR Alliance seems to benefit Southern with its incident response times. However, many alterations still seem to have to occur, with turnbacks at Barnham, Chichester and Fareham (et al) being required on numerous occasions.
