More than 40,000 rail workers are striking after talks failed to resolve a dispute over pay, jobs, and conditions.
Members of two unions are walking out, affecting rail services across the country - the latest industrial action adding to the country's transport woes.
Who is going on strike?
Members of the Rail, Maritime and Transport union (RMT) at Network Rail, Chiltern Railways, CrossCountry Trains, Greater Anglia, LNER, East Midlands Railway, c2c, Great Western Railway, Northern Trains, South Eastern, South Western Railway, TransPennine Express, Avanti West Coast, West Midlands Trains and GTR (including Gatwick Express), Heathrow Express, Lumo, Hull Trains, Grand Central, Transport for Wales, ScotRail, Merseyrail, Thameslink, London Northwestern Railway, Caledonian Sleeper and Stansted Express.
Members of the Transport Salaried Staffs' Association (TSSA) at Avanti West Coast.
The train operator I use is not on the list, so my travel won't be affected, right?
Network Rail said all train operators may be affected, whether they are part of the dispute or not.
Signallers, for example, control train movements across the whole country.
Will the London Underground be affected?
The industrial action doesn't involve workers at Transport for London, but there could be disruption on the lines that share track with Network Rail.
These are the District, Bakerloo, and Elizabeth lines, as well as the London Overground.
Some Elizabeth Line stretches will only have two trains per hour and on the London Overground a reduced service will operate between 7.30am and 6.30pm.
There could also be disruption the morning after the strike - 28 July - as things return to normal.
What if I do have to use the trains?
Passengers have been told to expect disruption and only travel if necessary.
But if you really are determined or desperate, be aware that trains will start later and finish much earlier than usual.
Only around 20% of services will run and national rail has urged passengers to check amended timetables - especially when their first and last trains will be running.
Some parts of the country will have no service at all.
What events could be affected by the rail strike?
The women's Euro 2022 semi-final is in Milton Keynes on the day of the strike.
The opening ceremony of the Commonwealth Games is in Birmingham the next day.
I'll just drive or fly
That's not going to be much fun either.
Roads are busy with summer holiday traffic - the Port of Dover had long queues through the weekend but Folkestone has since been called a holiday Hotspot of Hell by the AA. The usual culprits such as parts of the M25 and M5 are still best avoided during busy times.
As for flights, staff shortages at airports and airlines have resulted in thousands of flights being cancelled and delayed - with no end in sight.
What are the strikes about?
Network Rail's lead negotiator Tim Shoveller said the company had offered workers a two-year 8% pay deal with a guarantee of no compulsory redundancies, and other benefits.
He said the RMT had "walked away without giving their members a voice or a choice".
"Our door remains open to try and avert this pointless action that will cost strikers dear.
"We will now consider how we will move forward with our reform plans despite the RMT obstinacy."
The RMT said there has been no change or improvement in the pay offers it has received.
RMT general secretary Mick Lynch said Network Rail had "upped the ante, threatening to impose compulsory redundancies and unsafe 50% cuts to maintenance work if we did not withdraw our planned strike action".
"The train operating companies have put driver-only operations on the table along with ransacking our members' terms and conditions.
"RMT will continue to negotiate in good faith but we will not be bullied or cajoled by anyone."
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What do the train operators say?
A spokesman for the Rail Delivery Group, which represents the operators, said: "Sadly, these RMT strikes will upset passengers' summer plans, undermine businesses and upend the industry's recovery, making it more difficult to fund a settlement.
"We want to give our people an increase in pay, but we have a responsibility to do that by making reasonable changes to long outdated working practices - already successfully introduced in some parts of the network - which will improve punctuality, reliability and passenger experience.
"The alternative is to ask passengers to pay more when they too are feeling the squeeze, or asking the taxpayer to contribute even more towards the running of the railway on top of the record amounts spent keeping trains running during the pandemic and with revenue still 20% down on pre-COVID levels. Neither of those options is fair.
"Rather than going ahead with these counterproductive strikes, we ask the RMT's leadership to continue talking so we can come to a deal that works for our people, our passengers and for taxpayers."
And the government?
A Department for Transport spokesperson said: "The rail industry has to modernise and be brought into the 21st century for the benefit of passengers and staff.
"We're extremely disappointed to see that instead of staying at the table, RMT executives have chosen to walk away once more.
"We continue to encourage RMT to do the right thing by their members and passengers alike and call off the strikes."
If there is no breakthrough, what happens next?
Members of drivers' union Aslef at eight train operators will go on strike on 30 July. This will affect Chiltern, LNER, Northern, TransPennine Express, Arriva Rail London, Great Western, Southeastern and West Midlands Trains. Earlier this month, Aslef members at ScotRail voted to accept an improved pay offer from their bosses.
There are more RMT strikes planned for 18 and 20 August, and their London Underground members will strike on August 19.