Lines now expected to remain closed until end of service today (19th). Thanks to whichever member it was who edited the thread title!
Regarding the canal, I must say that the history of this particular route has always interested me and I am not surprised it is being called into question on here. I was well aware of the section able to be seen in Penge, etc. The sewer under the hole is active, but I would certainly not discount unstable old canal workings in the vicinity, either. The thing is, I do tend to struggle to find information about the canal and the engineering to convert it into a railway, partially (I should think) because this is one of London's older rail routes. Indeed the patterns of railway development in this area, and further towards both London Bridge and Crystal Palace, do basically remain fundamental factors in the layout of the lines, and therefore transport capacity, today - yet a lot of the evidence seems hard to glean at times. It does strike me that something could have dried up and contracted in the heat following so many patches of very wet weather recently, or that an old watercourse could gradually have been washing away material above, which had supported the track and was falling into it long before this issue arose, but I don't know if either of these are plausible in the absence of any evidence which I have seen. I'm extremely open to suggestions on these ideas as I am not a geologist and don't have any engineering qualifications relevant to this particular issue!