I took The Hornby OO scale Santa's Express Christmas Train Set (R1248) which is meant to go around the bottom a Christmas tree and built a platform for it to go around the middle of the tree above the reach of toddlers.
Brief and not so detailed assembly instructions on instructables here.
This is the second version of one that I did a few years ago mostly our of timber (available on instructables). This time it is 3d printable on a 230mm square bed.
Original SolidWorks files included. Not all relevant files would upload on this platform all can be found here on google drive.
Future developments may include N scale above, O scale below to give Three Trains in Chistmas Tree!
The author marked this model as their own original creation.