Rail workers at three train operators are said to be "solidly supporting" strikes, in bitter disputes over the role of guards and driver-only trains.
Members of the Rail, Maritime and Transport union (RMT) at Southern, Merseyrail and Arriva Rail North walked out for 24 hours, disrupting services.
Another strike will be held on Monday, while the Merseyrail workers will also stop work on Sunday.
The Southern dispute started more than 16 months ago, with the RMT taking more than 30 days of strike action.
The threat of industrial action against the industry's newest franchise holder is also looming.
RMT members on South Western Railway are to be balloted for strikes, just days after the operator took over the running of services from London Waterloo.
Paul Plummer, chief executive of the Rail Delivery Group, which represents train companies and Network Rail, said: "Rail companies are working hard to upgrade the railway to support passengers and our economy now and for the long term, including 6,400 more services a week by 2021, whilst opportunistic strikes by the RMT leadership seek to disrupt, as they did during the Grand National and Tour de Yorkshire."
Southern said it planned to run most services despite the strike, although there will be a number of cancellations.