Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online.The standard gauge (also named the Stephenson gauge after George Stephenson, or Normal gauge) is a widely-used rail gauge. Approximately 60% of the world''s existing railway lines are built to this gauge (see the list of countries that use the standard gauge). The distance between the inside edges of the rails of standard gauge track is 1,435 mm (4 ft 8+1 2 in). As railways developed and expanded one of the key issues to be decided was that of the rail gauge (the distance, or width, between the inner sides of the rails) that should be used. The eventual result was the adoption throughout a large part of the world of a "standard gauge" of 1,435 mm (4 ft 8+1 2 in), allowing inter-connectivity and the inter-operability of trains.