'5. Roadbed
There are many, many ways of installing track, but one of the simplest -- and to my mind, the best -- is to do it the same way a full-size railroad does it: float the track in ballast. Dig a shallow trench, say 50 to 60 mm deep, and fill it with ballast up to the level of the bottom of the ties. Set the track in place, making sure it is straight and level. When it is just where you want it, backfill with more ballast up to the top of the ties, which should be flush with the surrounding ground. There is no need to line the trench with plastic. On the contrary, good drainage is important and an underlayment may impede it.

By floating the track in the ballast, you are allowing it to move, which it will do. The heat of the summer will cause the rails to expand and the cold of winter will make them contract. The spring thaw may bring frost heave. Floating track will move as it needs to, but will be very easy to realign when necessary (probably just once or twice a year, if that). The more established the railway becomes, the less it will change.
The ballast you use is important. Don't use round stones like pea gravel. They will slide around and won't really grip the ties. A heavy rain will wash them away. Use a 4 to 10 mm crushed stone that has a "tooth" to it. This will lock with the stones around it and make a firm bed for the track to sit on. Crusher fines from a rock yard, or chicken grit from a feed store are two good sources of ballast.