During the 19th century, Spain was repeatedly torn apart by the Carlist Wars, a series of civil conflicts fought between royalist supporters of Infante Carlos and liberal defenders of Queen Isabella II. As casualties mounted and severe shortages plagued the Iberian Peninsula, international relief organizations and foreign allies attempted to transport medical supplies, food, and military aid across Europe to Spanish front lines. However, these supply chains hit an impenetrable bottleneck at the Pyrenees border due to conflicting railroad engineering standards.
Standard European railways utilized a track gauge of 1,435 millimeters (4 feet 8. 5 inches), whereas Spain intentionally adopted a wider Iberian gauge of 1,668 millimeters (5 feet 5. 7 inches).
Spanish military engineers had deliberately pushed for the broader gauge out of fears that a hostile nation, particularly France, could easily invade Spain using existing railway networks. While this defensive strategy prevented swift enemy troop rail movements, it proved disastrous during humanitarian emergencies. Incoming supply trains carrying vital aid from France were forced to halt abruptly at border stations like Hendaye and Irun.
Every crate of food, medical equipment, and ammunition had to be manually offloaded from French railcars and reloaded onto Spanish trains—a painstaking process that took days and required massive physical labor. During periods of severe conflict, cargo accumulated in border yards, where delicate medical supplies spoiled and crucial military relief was delayed for weeks, directly impacting casualty outcomes on the battlefield.