Signal workers, members of RMT, have been involved in two weeks of strike action over the imposition of new working conditions.
A mass picket was held in Newport on 8 January.
The RMT is organising an overtime ban and are balloting other signal workers to join in the action.
The dispute erupted when Network Rail tried to impose new rosters in a cost-cutting exercise associated with the opening of the new South Wales Control Centre this year.
