Trains usually use left-hand running on double track lines in part of the basic Spanish rail network and in the Madrid and Bilbao metro systems (as is the case with Portugal, France, Great Britain and many other countries); trains use right-hand running in most of the Spanish network and in Germany. This work analyses the advantages and disadvantages of running on each direction, the attempts at unification, the changes introduced throughout time and the current situation.

A detailed study about double track on the basic Spanish rail network (both conventional and high speed) is included, analysing the track duplication process, the current situation regarding the supply of double track, the direction of travel on this network, the existing track-to-track centre line distance, and the influence of reversible working and modern operation systems on the direction of travel.

This document has its origin in a communication presented to the 4th Railway History Congress (Málaga, 22 September 2006) which was published in the CD with the Congress proceedings and on the website: http://www.docutren.com/HistoriaFerroviaria/Malaga2006/. For the editions 2 to 4, this communication was corrected, extended and visibly updated addressing new topics. The fifth edition has some extensions and corrections as well as updated data until December 2016.