Cedar Point officials say one of the coaster's trains was ending its ride, when it rolled into a stopped train at the loading station.
Three people were taken to the first aid center.
Two were treated and released. The third passenger was transported to an area hospital for an asthma condition.
The Magnum was back open as of Sunday.
In Dayton, Ohio, another three people were injured on a kids' ride at RiverBlast. Witnesses say the ride jolted forward as children were exiting, dragging some along in a circle until the ride was shut off.
A 6-year-old girl and a 10-year-old girl were taken by ambulance to Children's Medical Center of Dayton, where they were treated for cuts and abrasions.
Another 1-year-old was brought to the hospital by a parent.