Industrial action by the RMT union kicked off this morning, affecting Southern rail, Merseyrail, Arriva Rail North and Greater Anglia services.
The strike, which relates to disputes over the role of guards on trains, is set to continue on Thursday, when London Underground workers are also planning a walkout.
This is what London commuters need to know about today's strike:
Southern will run a normal service on most routes, however there will be some alterations
A reduced service will run on the following routes:
Clapham Junction - Watford Junction (A limited service will operate between these stations at peak times only.)
Southern services to and from Guildford will not run, meaning passengers will need to use South Western Railway services. However, these are currently subject to disruption caused by a platform closure at Waterloo, so careful planning may be required.
Greater Anglia are reassuring their customers that a full, normal service is planned to operate on both strike dates. The rail operator said additional staff have been trained to cover the conductors roles.
However, RMT is planning further action due to its guards dispute with Greater Anglia - conductor members of the union have been instructed not to take on any overtime or rest day working from Tuesday 10 October to Monday 6 November.