Use London Bridge rail or New Cross Overground stations? Then you could be in for problems with your trains at the end of this month.
The interminable Thameslink programme at London Bridge, upgrading the track and station in a bid to end the sodding bottleneck, hits a new flurry of activity between 23-31 August. Platforms 12 and 13 are going to be re-opened but platforms 10 and 11 will be closed for demolition and rebuilding. During this period, First Capital Connect trains won't call at London Bridge, being diverted between Blackfriars and East Croydon with some services stopping at Elephant and Castle. Southern trains will have fewer platforms available so is running an amended service. Southern's normal timetable also changes on 1 September to take the new configuration into account, so check your usual journey. There will also be no Overground services from New Cross between 26-29 August.
This is particularly going to hit you if you're travelling to Gatwick. Make sure to plan ahead so you're not caught out at London Bridge with a ton of luggage (the interchange at Elephant and Castle is not fun; go to Blackfriars).
While we're on the subject of London Bridge, here's another reminder about the major Southeastern changes from January 2015 because of Thameslink work. Trains to and from Charing Cross will not stop at London Bridge until August 2016 and the Greenwich line, New Cross and St John's will lose their direct connections to Waterloo East and Charing Cross forever — all trains will be routed to Cannon Street.