Port Talbot railway signal workers begin strike.
The Rail Maritime and Transport union said the walkout by workers in the Wales and the Marches area, including those in Port Talbot will hit services, although Network Rail denied there will be any disruption.
The union claimed managers standing in for the strikers had received five hours of briefing for duties that would normally require a minimum of a month's full training.
As well as Port Talbot, workers taking action are based in Cardiff, Newport, the Vale of Glamorgan and Rhymney Valley.
RMT general secretary Bob Crow said: "Our members are rock solid in their determination to stop the bulldozing through of rosters at the new South Wales control centre which we believe are all about saving money and which unilaterally rip up existing agreements.
"This is nothing less than an attempt by management to impose working conditions that will allow them to shove staff around at will and force them to work up to 13 days back to back to cover for vacancies and save Network Rail money. That goes right to the heart of this dispute.
"RMT maintains that the existing 12-hour roster is tried and tested and that the 8-hour roster that Network Rail are trying to impose at the new South Wales Control Centre when it opens in January will have damaging consequences for both staff and the service. Work-life balance arrangements will be wrecked in a drive to make financial cuts.
"Rather than taking risks with safety by drafting in under-trained managers from other parts of the UK in an effort to break this strike it would make much more sense for Network Rail to get back around the table to negotiate a settlement to this dispute."
An NR spokesman said: "We have enough trained managers and supervisors to run a normal service in and around Cardiff if the signallers' strike goes ahead."
The strike is due to end at midnight on Saturday.
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