I've said much on the subject and managed track / lineside drainage through some of the wettest years on record WITHOUT it resulting in catastrophic slips etc, despite the neglect that existed in off-track maintenance during the 1992 -2000 period so feel I know something about the reality of the issue and maintenance does play a HUGE part in dealing with it and it's not like NR can say they are unprepared because they've had a whole department dedicated to the weather and its effect on the infrastructure since the late 90's (Railtrack era included). I've attended more pointless EWAT talking shops that I care to remember in the past - an quite honestly it became a refuge for many that were no good at anything else.
It is marvellous having the high engineered track systems of today - fastclips, concrete bearer S&C and the systems to install it etc etc - it all looks nice and enables savings in Patrolling and Production staff costs etc. - but if it's laid on poor formation due to drainage issues or not adequately maintained (see GWML) it is meaningless.
An emerging issue in recent years is definitely that there are areas now where drainage is required where maybe none existed before but that's still no excuse for letting the existing systems deteriorate further.
Just my subjective opinion as an experienced PW Engineer not in the front line anymore. I wouldn't couldn't be within NR these days and can only wonder at how people are struggling now ............................
and as usual I like to finish with a "war story"
Back in oh say, 2003-ish, I was responsible for the Leatherhead to Bookham / Dorking lines which are both on embankments south of L'head - the Bookham line turning sharp R and the Dorking continuing straight - there is (or was) a s large tract of land in the vee (not railway owned ISTR) but drained apparently by 6 culverts into the River Mole (this river has been forever entwined in my late NR railway life) - except it wasn't and had become a large lake. Enter the Mole Valley District Council (or whatever they were back then) "NR - we want this drained - can you make it work" ?
"No problem - sez I"
Just before we were going to set about jetting and clearing these culverts to let the backed up water flow into the Mole somebody had tipped off the NRA and a very earnest young lady (where do they find them) who demanded a site meeting which we had - at which she pointed out that said "lake" water had been there a while and was effectively stagnant and if we thought we were going to clean out the culverts all at once and dump this stuff into the Mole eco-system NR would pay dearly for it (possibly in blood as it seemed at the time).
Anyhow a suitable compromised plan of phased culvert cleaning was approved and happened and as far as I am aware we didn't poison half of Surrey in the process - neither did the embankment fall down even though it's feet has been unnecessarily saturated in water for ages. The land is probably housing now.