Hey Alex,
Does Dad allow you to assemble and run his trains on the carpet on your own? Can you use one of his sets?
Building a layout is not a simple as it appears. It takes time. You should demonstrate to the parents that you have the time to dedicate to the process and not let your school work, sports and other commitments fall short. Also, that you can and will complete the project.
Is your grandfather handy with tools and fixing things? Can you enlist him as a partner? Can Dad/Mom spare any time to assist?
The first lesson will be in planning a project.
We look forward to your responses. You'll find a lot "train friends" willing to help you.
Good Luck!