The reason is quite obvious if you know Spain's History. Given that France invaded Spain in 1807, the military made it necessary to have a different gauge from

The reason is quite obvious if you know Spain's History. Given that France invaded Spain in 1807, the military made it necessary to have a different gauge from France. Not only that, the train by the coast was also forbidden in some places as a naval bombardment could disrupt communications in case of war. Spain has lots of mountains with a large plateau over 700 meters high and the coast is usually way lower so it makes sense to transport things by the coast.