I like this tutorial because it uses common materials and works for almost any kind of train: Making detailed oaks and maples.
It doesn’t hurt that the results look good too.I don’t think there would be anything wrong with using these trees on a tinplate prewar O gauge, Standard Gauge, or 2-inch Gauge (Carlisle & Finch and the like) layout. I don’t know if hobbyists were using these techniques 100 years ago, but the materials were all available then.
Lichen is available at craft and floral supply stores. You could also buy floral wire there to use in the project in place of electrical wire. Plaster is available at craft and hardware stores. And you can paint it with craft acrylics, available at craft stores and even some discount stores.
A lot of projects require a good hobby shop close by, and not everyone has one of those anymore. Living where I do, there are three good shops within 15 minutes of home (one is only about three miles away) but some projects require specialty items those stores don’t have. About a year ago I went to the estate sale of a model railroader who was extremely good at building and superdetailing kits. Virtually everything he had came from a hobby shop much further away.
The first thing I thought when I saw this project was that I could probably go out and buy everything I needed to make some of these even if I was visiting my in-laws in southeastern Missouri. And I might not even have to drive 30 miles to the nearest Hobby Lobby to get what I need.
It might be the first project I’ve ever seen where this is true.