Scenery and terrain

If you have planned your track layout in detail, using software or the old-fashioned (pen and graph paper) approach, it should be possible to construct the terrain and scenery independently from laying track. If, like me, you don't have that degree of self-organization, the landscaping process will overlap to a certain extent with the track layout -- as can be seen in the photos below.

Construction is much easier in general if the layout is all on the same level. However, there are more interesting scenic possibilities if there are multiple levels, as it opens up possibilities to include bridges, viaducts, and the like. If you are using multiple levels, then you will need to provide ways for the trains to get from one level to another. This will require either a ramp -- which can be integrated into the scenery as a hill -- or a helix, which can't. A helix is simply a spiral of track, which climbs the required distance in a number of loops. This allows a gentle gradient to be maintained while climbing significant distances. However, they are difficult to construct and have to be hidden, as a helix is not a structure that occurs in real railways, and would look a bit daft.

The problem with ramps is ensuring that the gradient is not too steep. Clearly, the shorter the ramp, the steeper it will be for a given distance. A rise of about four inches per six-foot length will be OK for most trains, and won't look too extreme, although climbs this steep don't really occur all that often in real railways.

So, the next step in construction was to assemble the main raised sections and ramps. Notice that there is already some track in place; it is difficult to determine how well trains will run on ramps and curves like these, and the best way to be sure is to try it.

The raised sections are constructed in the same way as the main base: fibreboard on wooden supports. For short and irregular sections -- like the switch-back -- the supports are made of fibreboard as well, as it is easy to cut to shape. In the photo below, the large books resting on the fibreboard are there to hold the two layers of fibreboard together until the glue sets.

It's worth bearing in mind that using multiple decks does not necessarily gain you any space, because you can't really run trains on both decks, one track above the other. This is because you to retain access to all the track for cleaning and maintenance. For short runs of track -- such as where the lower track runs in a tunnel below the upper deck -- you can get away with it provided that your hand can squeeze into the tunnel with a cleaning block.

My original plan was to have the inclines between the upper and lower decks nearly vertical, as would be the case in a quarry area, for example. However, as work progress I found that I preferred a more `rounded', rocky look.

Terrain

A model railway is more interesting, and more realistic, if the terrain is not perfectly flat. A few hills make all the difference, particularly if there are multiple track levels. There are a number of very common methods of making rocky outcrops and rock faces. The traditional technique is make an frame of chicken wire, then coat it with papiere mache. I understand that it works quite well, but the process seems too messy, and the result is likely to be quite heavy. Another popular approach is to built layers of polystyrene foam cut roughly to shape, and then carved when the layers are glued together. A number of manufacturers, notably Woodland Scenics, have proprietary landscape-building systems; these look quite effective, but are rather expensive.
The approach we have adopted is simple and cheap: newspaper and plaster-impregnated cotton bandage. The basic structure is made up from wads of newspaper, then the plaster bandage is soaked in water and laid on top. When the plaster dries it sets into shape. The photo below shows the newspaper wads ready for the plaster to be applied (foreground), and the structure after plastering (background).

Then the bandages get a coat of brown paint. We have used a number of different shades of brown, anticipating that some of the paint will show through the covering material. Whether this is important or not depends on what the structure will be covered with.

Then the structure is covered with grass material; Woodland Scenics make a nice range of grasses. Again, you'll need multiple shades to get the best effect. Finally, we've put on some lichen to give a bushy effect, and added a few trees. These are made from lichen glued onto twigs from the garden.

There are a number of things to watch out for with this technique.

The cotton forms a mesh to which the plaster adheres; a single layer of cotton will be rigid, but the holes in the mesh will be very visible. This means that you'll need several layers to get a hole-free structure. Whether this is important or not depends on what the structure will be covered with. If it's to be grassed, or covered in foliage, it doesn't matter; if it is just going to be painted or dusted with ground scatter it matters a lot.
If you are using multiple layers -- either to eliminate the pores in the mesh or for extra rigidity -- be sure to press the layers together firmly. Otherwise you'll end up with lots of single layers separated by air, which is neither stronger nor less holey than one layer.
When dry, the whole structure is somewhat brittle. It is not too difficult to poke a screwdriver or a finger right through it if you're careless.

Ground cover

You can get basic `ground' effects simply by painting the baseboard the right colour. The problem with this is that real ground is usually not perfectly smooth. Greater realism can be achieved using a variety of techniques. The most commonly used are `scatter', flock, and matting. Of course you can, and probably should, use all these techniques together.

Scatter materials

`Scatter' is generally sawdust, dyed to a particular colour. You can get various shades of green to represent grass, brown and yellow for soil, gray for gravel, etc. If your baseboard is wood, you probably won't be able to put scatter onto it directly, unless you want to use a really thick layer. As a minimum, you'll need to paint it with a base colour first.
My opinion is that scatter does not make for very good ground cover, except in small areas. The problem is that it looks like scatter. It is, however, dead cheap, especially in big bags. If you paint a hill green, then sprinkle a few shades of green scatter on the wet paint, you get a basically serviceable grassy hill. There are better techniques, however.

Flock materials

Flock is short, fine fibres. Like scatter, it is available in a range of colours. Unlike scatter, it looks quite realistic, especially when used in a range of shades. There are two problems with flock: it is tiresome to apply, and it is very expensive. If you have large areas to cover, you may be better off with matting.

Matting

This is paper covered in a thin layer of fibres or colour sawdust. Good matting is quite expensive, but covers the area really quickly. It doesn't look as good as flock, in my opinion, but is much easier to use. You probably won't be able to use matting exclusively, as the cut edges don't look very realistic.
Matting is available to simulate grass (various shades), gravel (good for car parks and industrial areas) and mud.
You can stick matting so that it is perfectly flat, but left to its own devices it tends to wrinkle slightly. This is actually a good thing: real ground is rarely perfectly flat.