A floor structure is provided with an underframe, which is part of a carriage body, is elongated along a longitudinal axis coinciding with the advancement axis of such carriage, and has a plurality of longitudinal attachment rails arranged on an area of the lower surface of the underframe; the attachment rails project downwards so as to define therebetween a plurality of compartments, which are engaged by panels having insulating material to form a fire protection system; such a system has, furthermore, a plurality of section bars, which are attached to the attachment rails and are shaped so as to retain the panels in fixed positions in the compartments and, at the same time, cover the attachment rails.