The crash on the £18mln ride resulted in two teenagers needing leg amputations.
Victims told Stafford Crown Court how they watched in “horror” before they ploughed into an empty carriage that had stopped on the track midway around the 14-loop ride. Prosecution likened the impact to a 90mph car crash.
The court heard that an engineer “felt pressured” to get the ride back into service after it developed a fault during a busy period.
In April, Merlin Attractions Operations Ltd admitted breaching the Health and Safety Act.
Alton Towers theme park in Staffordshire, England, initially blamed the crash on human error, but prosecutors argued the fault was with the employer and not individuals.
Judge Michael Chambers QC ruled the crash as a "catastrophic failure" and said human error was indeed not the cause.
"This was a needless and avoidable accident in which those who were injured were lucky not to be killed,” said Chambers.
Merlin chief executive Nick Varney stressed the firm was not an emotionless corporate entity.
"In this context, the far greater punishment for all of us is knowing that on this occasion we let people down with devastating consequences," he said.
"It is something we will never forget and it is something we are utterly determined will never be repeated."
The Smiler rollercoaster was back up and running in March this year. It is described on the Alton Towers website as “the biggest ever investment for a ride at the resort” and promises “a complete physical and mental assault on riders”.