Strikes by workers at two rail rail companies in the south over the role of guards and driver-only trains are being "solidly supported", according to the RMT.
Members of the RMT union at Southern and Greater Anglia walked out for 24 hours, with another stoppage due on Thursday.
The RMT has written to Prime Minister Theresa May, accusing her of "sabotaging" negotiations aimed at resolving the disputes.
- Greater Anglia is planning to run a full service on the strike days
- Southern said there will be a normal service on most of its routes