Are the railings 6' outside to outside or center to center.?

Hello, and thank you for your interest in Trex! Also thank you for your inquiry as that is a great question due to the fact that Trex manufactures varying railing lines where some are designed at on center measurements, and some are designed at clear span measurements. The Enhance railing line shown is manufactured at clear span measurements which means the rail length would be 72" in length. I hope this helps! Thanks, Tori - Trex Customer Care Team