Règle correspondante
Practice Relating to Rule 8. Definition of Military Objectives
Section F. Lines and means of transportation
Spain’s Field Regulations (1882) stipulates that bridges and railway equipment are legitimate objects of attack. 
Spain, El Reglamento para el Servicio de Campaña, 4 January 1882, § 880.
Spain’s LOAC Manual (2007) states that military objectives include:
[C]ivilian vehicles, ships and aircraft when they take part in hostilities, transport troops or military supplies, are escorted by military personnel (unless there is an agreement to the contrary), cross duly marked prohibited areas (unless there is an agreement to the contrary), disobey an order to stop or actively resist inspection. 
Spain, Orientaciones. El Derecho de los Conflictos Armados, Tomo 1, Publicación OR7–004, (Edición Segunda), Mando de Adiestramiento y Doctrina, Dirección de Doctrina, Orgánica y Materiales, 2 November 2007, § 4.5.b.(2).(a).