The first of several strikes is disrupting train services seriously, because ASLEF drivers have walked out at nearly all train operators in England, with the exceptions of c2c, Merseyrail, Transport for London, Glasgow Subway and Tyne & Wear Metro. Many stations served by the affected operators have no trains at all today.
Where trains do run, timetables will be limited and services will cease towards the end of the afternoon.
Neither ScotRail nor Transport for Wales are directly affected, although trains to and from England are being disrupted.
RMT members at train operators are set to strike tomorrow, and more ASLEF strikes are planned for 31 May and 3 June.
ASLEF is also staging an overtime ban on 13 and 15-20 May, and again on 1 June.
The latest round of industrial action, in continuing disputes over pay, comes after RMT members had voted in favour of continuing strikes up to November.