Southern Railway is planning to run a normal service on Tuesday despite a strike by a small number of drivers.
Members of the Rail, Maritime and Transport union will strike on Tuesday, Wednesday and Friday in a dispute about driver-only trains.
The main drivers' union, Aslef, has called off its strikes and ended an overtime ban and is continuing talks with the company at the TUC, without the RMT being invited.
Talks between Southern and Aslef are due to resume on Tuesday.
RMT members working as guards went on strike on Monday in a separate row over the role of conductors, causing disruption to almost a third of Southern services.
General secretary Mick Cash said: "RMT is a recognised union for drivers on Southern and it is disgraceful that we have been carved out of the current talks process set up by the TUC and the Government. That is a kick in the teeth for our members.
"RMT has repeatedly requested access to the talks process involving the TUC, the employers and our sister union but we have been roundly snubbed and our executive committee finds that wholly unacceptable.
"All parties should be at the negotiating table and RMT's continued exclusion means we have no option but to press ahead with our industrial action."
The RMT, which has 12 driver members, will mount picket lines at some stations.
It comes as a report showed house price growth along Southern routes has been less than half that across the UK since October, which is blamed on the industrial disputes.
Online estate agent HouseSimple.com said average increases for houses along the Brighton mainline and other Southern routes was 1%, compared with the national figure of 2.5%.
Chief executive Alex Gosling said prospective buyers had "understandable concerns" over the reliability of Southern services.
Elsewhere, South West Trains passengers were hit by cancellations and delays because of a broken rail at Wimbledon, and talks resumed to try to avert more strikes on London Underground over jobs and ticket office closures.
The RMT and Transport Salaried Staffs Association staged a 24-hour strike earlier this month which crippled Tube services and caused disruption in the capital.
Manual Cortes, general secretary of the TSSA, said: "The clock is ticking and we need an agreement by the end of this week or, sadly, further strikes will follow.
"Our negotiating team remains committed to reaching a resolution which ends this long-standing dispute."
Mr Cortes accused former London mayor Boris Johnson of an "act of vandalism" in agreeing to cut hundreds of jobs and close ticket offices.
The Tube talks have been adjourned until Wednesday.