Various other local operators were warned this afternoon that this could be brought forward, and thus it was so. The Core seems to have been closed Southbound from around 1655 and Northbound a little after that. I can't get more details right now other than that Thameslink have declared CSL2 Code Black (ie. the most severe disruption level) and tickets are accepted left, right and centre on a number of routes.
Given the timing of when services will resume, I can only presume that stress on some key component or system reached a level where it was unsafe, but that the work to repair this would only take a relatively small amount more time to complete now as it would later on. Hopefully the time of normal services resuming has been designed to take into account any availability of engineers and equipment earlier in the evening.
EDIT: Latest news that I have just found is that it looks like trains will be amended up to and until at least the time when 2W82 07/10/2015 0010 ex Brighton reaches London Blackfriars (in reality around 0130ish), where it will likely terminate.
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More info: Disruption is now expected until 0600 tomorrow (Weds 7th).
