Out of my (Williams Free) collection my "Go To" engine for what you want to do would be my Lionel Mikado Jr. I have the Union Pacific version, and they are Great Looking, SMooooth Running and Strong pulling locomotives. I would have no concerns running a Mikado Jr. for extended periods, and you would need a MUCH larger area than that to come close to the length of train that they can handle.
This would be my choice, but the Lionel 0-6-0T Docksider would also be another Great choice in my opinion.
Doug