It should never have happened.  I cannot see how there could be any excuse for sending a carriage round with people on when there was a test carriage still on the track in front.

It seems an onlooker even warned them about it but was told to mind their own business.

http://www.itv.com/news/2015-06-04/alton-towers-visitor-who-warned-staff-about-carriage-on-rollercoa...

So sad for young Leah who has lost a leg thanks to their negligence.  They have owned up to full responsibility and will pay compensation, but that's little consolation for that poor girl and others badly injured when the accident could have and should have been avoided.