Drivers from three more train companies are set to go on strike later this month, worsening already expected rail disruption.
Drivers working for Hull Trains, Greater Anglia, and Croydon Tramlink have all voted to take industrial action in separate disputes over pay, union Aslef has announced.
Greater Anglia is the owner and operator of the Stansted Express, a direct service between the airport and London Liverpool Street, meaning strike action could cause misery for holidaymakers.
It comes as the RMT union said up to 50,000 of its members across Network Rail, 13 train operators, and the London Underground would walk out on three days in June - the biggest outbreak of rail industrial action since 1989.
Taking place over three days, the strike action threatens to "shut down the system".
Talks between Network Rail and the union are expected to be held in the next few days, with the former drawing up contingency plans to try and ease the disruption to services.
Fewer than one in five trains are likely to run, and only between 7am and 7pm, probably only on main lines.
The majority of the strike action will take place over a nine-day period at the end of June.
The first RMT walkout is scheduled to take place on 21, 23, and 25 June. These strikes are expected to cause disruption to services for six days.
Meanwhile, members of Aslef on Hull Trains will strike on 26 June, at Greater Anglia on 23 June, and on Croydon Tramlink on 28 and 29 June 28 and 13 and 14 July.
Third union could join 'summer of discontent'
Members of a third union, TSSA, could also join the strikes, further compounding the travel chaos.
TSSA has served notice to ballot hundreds of staff at Avanti West Coast in a dispute over pay, conditions and job security.
Strike action at the train line could take place as soon as 13 July, if it is voted upon by the union's 300 members.
TSSA General Secretary Manuel Cortes said: "We could be seeing a summer of discontent across our railways.
"Make no mistake, we are preparing for all options, including coordinated strike action. And if our members go out on strike in Avanti, the trains will stop running."
Avanti operates passenger train services and stations including from London Euston to Birmingham, Crewe, Liverpool, Manchester, and Glasgow.
Ticket offices that 'barely sell a ticket a week'
Speaking in Blackpool today the prime minister said the "time has come" to close inefficient train ticket offices across the UK and replace them with automated systems as part of cutting the costs of transport.
"We are on your side in cutting the costs of transport, not just with the fantastic investments that we are making," Boris Johnson said.
"It is time for us to grasp the nettle of reform and move sensibly and responsibly to end some outdated working practices.
"There are fully-manned ticket offices in this country that barely sell a ticket a week."
He described the closure of ticket offices on the London Underground in favour of automatic ticketing under his tenure as mayor of London as "initially painful" but added: "We successfully made the argument that staff were better and more productively deployed on the platforms interacting with the public.
"The time has come to do the same thing across the transport network."
Labour shows support for rail workers
Earlier today, shadow leveling up secretary Lisa Nandy said she supports rail workers but stopped short of giving direct support for the planned strike action.
"I've stood with our rail workers just like I stood with junior doctors when they protested against the treatment that was being meted out to them by the government, and our nurses as well," she told ITV.
"The way that you create good public services is not to attack the people who run those services, is not to attack those people who work day in, day out in order to try and keep them going - the way you do it is to support them."