Once you’ve built your line, you’re going to want to show it off!
The best way to do this is to host an open day. Invite friends and family, or post up an invitation online – but I would advise you not to disclose your address until people have, at the very least, emailed to request it. This will keep out most unwanted visitors.
On the day, people may wish to bring their own trains to experience having them run on your line, you should specify any restrictions in advance (for example if you do not wish to allow steam trains on safety grounds) so that people are not disappointed when they arrive.
Draw up rules for safety (including such things as not crossing the line without looking, not driving the trains without permission etc). Even if you think these things are just common sense, putting them on paper can solve a lot of problems later. Ride on Railways has an excellent section dealing with safety.