PARIS Trade unions at France 's railway operator SNCF has called for an indefinite strike from next month that could disrupt train services during the upcoming Christmas holidays. The unions are demanding a moratorium on the dismantling of Fret SNCF, the freight division of the national rail operator, and protesting against the terms and conditions for opening up regional lines to competition. In a joint statement, the CGT-Cheminots, Unsa-Ferroviaire, Sud-Rail and CFDT-Cheminots unions said on