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By Rachel Russell
BBC News
More than 45,000 rail workers have begun another day of strike action in a dispute over pay, jobs and conditions.
Network Rail said only 20% of services will run with train companies LNER, Cross Country and Chiltern Railways among those who will be affected.
Passengers saw disruption on the rails in a strike two days ago and the London Underground on Thursday as RMT, TSSA and Unite union members walked out.
The RMT union said a "stalemate" means the dispute may last for a long time.
It has accused the government of blocking the train companies from offering its workers a better deal.
However, ministers responded that the unions were causing the block, adding that if the deals on offer were put to the workers, they would likely accept them.
Which trains are still running?
Only 4,300 services out of the usual 20,000 across the UK will run on what is now the sixth day of national rail strikes this summer.
In London, the bus strike will continue on from Friday, with routes being particularly affected in south-west London and parts of Surrey.
ScotRail said the dispute did not involve its staff, but there could be a knock-on effect in Scotland as there will be just 11 routes open.
Because rail strikes often leave trains in the wrong part of the country at the start of the next working day, Sunday travellers have been warned to expect some disruption - although 85% of services are expected to run as normal.
Which events will be affected?
Music and football fans are advised to check whether they will still be able to travel by train to concerts and sports fixtures - or whether they need to switch to travelling by car or coach.
The day's big music events attracting thousands of travelling fans will include Becky Hill at Crystal Palace Park in London and the All Point's East festival, in Victoria Park, London. Camp Bestival is taking place in Shropshire but it is thought that most of its 90,000 ticket-holders will already be on site halfway through the four-day festival.
And of course every Saturday during the football season sees a mass migration of away fans.
Wolves fans will be travelling in their thousands to London for their lunchtime match at Tottenham while fans of some big Championship clubs such as Sunderland are having a second awayday this season disrupted by the strike action.
Why are the strikes still ongoing?
The unions are in dispute with the government and rail companies as they say salaries should increase to reflect the rising cost of living.
Network Rail, says its latest offer is worth more than 5%, although this depends on whether workers accept "modernising reforms".
However, the RMT says this is a "paltry sum".
Mick Lynch, general secretary of RMT, told the BBC: "We can deal with evolution of the railway, but what we can't have is imposition of change and detrimental changes to our members lives that will make them not able to change their bills."
Dan Panes from the RDG also told the BBC the aim was to get "conversation rather than confrontation".
What other strikes took place this week?
Staff at Network Rail took action on Thursday with rail strikes, while, while TfL said the Tube strikes on Friday resulted in usage of the underground being 90% down on the same day last week, with 98,345 entrances and exits on the LU network up until 10AM.
General secretary Mick Lynch apologised for disruption to the Tube, but claimed the union had been shut out of talks between the government, the Treasury, and Mayor Sadiq Khan's office.
Mr Lynch said: "They've got to get us around the table so we can talk about their issues, but at the minute there's been a big stalemate."
In response to Mr Lynch's claims, Mayor of London Sadiq Khan said he and the RMT union were "on the same side here, nobody wants the government to be attaching unreasonable conditions to our deal".
A spokesman for the Department for Transport accused union leaders are "opting to inflict misery" by disrupting travel.