Thanking FirstGroup staff, Jamieson, who came second in the 200m breaststroke at the London Games, said: "I had been to London and was going home quite late. I just got up and walked off the train without it. I never thought I would see it again - but it's great to finally have it back."
The swimmer was reunited with his luggage, which included a Team GB wash bag and tracksuit bottoms, at Bath Spa station on October 19.
Station manager Peter Rignall said: "It's so pleasing to be able to help this young star out and have his case returned to him."
Sean Duffy, ScotRail's commercial director, said: "We saw Michael's tweet asking for help and are delighted to have reunited the silver medallist with his Olympic belongings."