My garden is rather flat, while great for roadbed, there is no natural terrain of any interest.
To block sight lines, I placed a large mountain range in the center of the space granted me for the railroad. I started the range by using well-spaced cinder block to define the height and length of each arm. Pipes for the rivers coming off the mountain peaks were installed.
Next, about 15 yards of fill dirt were piled up to form the slopes of the mountains. This sat for a couple of winters with little settling.
Field stone (free from any farmer in this area) was placed on top.
Several inches of compost were added to cover some of the stone and provide for planting. This did settle and wash away easily, so I expect to be dressing the slopes every spring.
Excavation of the river beds, harbor, and lake finally provided the interesting terrain that my roadbed will bisect and bridge.