No-one tells you when you come to visit London, or often as a Londoner, that every Saturday and Sunday there are weekend engineering works on the London Underground and many mainline train lines. There will be times you can hardly notice it happening and others when your simple half hour journey turns into a 2 hour marathon of replacement buses and alternative routes.
I suspect the schedule for the weekend engineering works is planned further ahead than we can find out but what you can do is check the Transport for London website at: tfl.gov.uk/check and also sign up for email alerts which arrive during the week, usually on a Thursday, for the weekend a few days away.
The email lists the engineering works by tube line so do check if the line you're staying on will be affected. But, obviously, and especially at the weekends, you are very likely to want to travel around London so you need to know about Journey Planner.
This is a part of the TfL website that helps you plan a journey on a specific date and time, and because it then knows when you want to travel it factors in any planned closures when offering routes.
There are also smartphone apps that can help you plan journeys but I've not found one yet that can offer me a mix of London Underground, buses, walking, overground trains and even river boats, in the way that Journey Planner can.
At the start I mentioned mainline trains. These are trains that travel from London to other destinations around the UK, such as to Oxford, Birmingham or even Scotland. These trains are not operated by Transport for London; instead each line is managed and run by a private company. Thankfully you can buy tickets for all the different company's trains at each mainline train station, or online in advance. (See National Rail Enquiries).
As these trains cover longer distances, and alternative train routes may not be available, the train companies provide rail replacement buses to link the stations along the sections of line that can't be used. There is no extra charge for these buses but it will make your journey longer.
In London, when a tube line isn't working in central London for planned engineering works, there won't be a replacement bus service as there are so many alternative ways to complete your journey. If you are not in central London, and there is no obvious alternative, i.e. not two lines from the same station and one still working, then a rail replacement bus will be made available.
The best thing is not to worry if your journey is affected but if it's a trip to the airport or somewhere important then do check first and allow extra time.
Why Does It Happen?
London Underground is the oldest underground rail network in the world so it does need maintenance and this generally has to happen on weekends. Yes, it can be disruptive but use it as an opportunity to try buses which can often be more direct, or enjoy walking which is one of the best ways to help orientate yourself in London as the tube map is more of a diagram than geographically accurate.
Use an A-Z, or the maps on your smartphone (you might need Pocket Wifi Rental) and also don't be afraid to ask a local for directions. Londoners can be very approachable and most will do their best to help if they can.