Simplify the following text for a Grade 7 native English-speaking person.
Original: In Great Britain , Stephenson 's gauge was chosen as the standard gauge on the grounds that lines built to this gauge were eight times longer than that of the rival gauge , adopted principally by the Great Western Railway .
Simplified: The standard gauge ( also called the Stephenson gauge after George Stephenson , or Normal gauge ) is a popular rail gauge . About 60 % of the world 's current railway lines use this gauge .