Thanks again. RMWeb is back up so you can check out my
Whitemarsh blog for some more construction details. It's a bit patchy because I tend not to write updates when I'm busy modelling, so if I have a good run of work on the layout the updates get very few and far between.
I've tried to collate some of the construction pics into a Flickr set (they were scattered all over) so I could do a quick rundown here. I've mixed in some pictures from other layouts when they illustrate the concepts better.
TerrainI like my railway layouts light, so I use very little timber and build the scenery out of extruded polystrene foam - in this case Styrofoam (the blue stuff) from Dow Corning. I like this a lot more than expanded polystyrene (white bead foam) because you can carve it, sand it, it's more rigid and less messy. (If you're in the US, be aware that Styrofoam cups aren't made from Styrofoam

)

Whitemarsh Yard
In this pic the main board is a 50mm slab of foam cut away at the front with a panel saw with 5mm foam card inserted to make a flat base for the water in the fen drain. There's a frame of 18mm PSE softwood underneath that to provide clearance for the wiring, but most of the rigidity comes from the profile boards front and back, which are like the web of a girder. They're made from 3mm MDF which is easy to cut with a Stanley knife and sands and paints up nicely (you need an undercoat though - it drinks paint).
The Styrofoam doesn't get much of a workout in this layout - it's really just structural and allows me to poke holes when planting things, but you can do a lot more as you can see here in another layout based on the Höllental (Hell Valley) in the Black Forest - this is all carved Styrofoam.

Whitemarsh Yard
(click for bigger)
If I need to fill in gaps, smooth contours, or otherwise build up the surface I use what I think is called "lightweight spackle" - it comes in tubs, smells like acrylic, and has tiny hollow glass beads which make it very light. It's really nice to work with - clean and soft. It sticks when you work into into a surface but won't make a mess even if you spill it on carpet or clothes - you can just pick it up. It doesn't shrink much and dries very slightly soft which lets you impress it with a hard tool to create surface detail, or you can stipple it with a brush when it's started to set up but not cured. I haven't needed much of it on this layout, but there's about a tub and a half in the German one fairing in around all the rocks.

Whitemarsh Yard
Over the top of that I like to apply a mix of gesso and acrylic paint. I use artists' acrylics in tubes since they have a lot of pigment, but house paint probably works. Gesso is a primer for canvases which usually has a mixture of plaster and an acylic binder like PVA. I usually mix in some fine sand and raw umber paint to get a grey-brown base. This one does shrink so you can't really use it as a filler, but it creates a tough, slightly flexible skin which won't crack or show the white or (god forbid) light blue underneath. Don't put the sand in when you're doing rocks because it usually looks silly, do put it in for the ground - it adds tooth to the surface and helps it hold the texture in the next step.

Whitemarsh Yard
TexturesOnce that's all done you've got some kind of scenic base, and the fun bit starts - covering it up. For earth, you can brush on neat or dilute PVA, or better yet matte medium, which doesn't shine if you don't cover everything up. Then sprinkle with a mixture of scenic materials - crushed dried earth, gravel and flock from model shops, bits of twig, etc. etc. I tend to make a mix for one purpose (like forest floors) and when I've used most of a pot of it, add in some more without following an exact recipe, so it varies around the scene but has an element of consistency too.
You can add quite a thick layer of texture, and move it around carefully with a soft brush when dry. When you have it how you like it, mist it with wet water (water plus a drop of washing up liquid) from an atomiser. You need very fine drops, sprayed from far back, so you don't make craters or blow it around. Once it's throroughly wet, dribble on a 50/50 mix of water and PVA with an eye dropper (pipette). You want to more or less saturate the texture. Then leave it for 12 - 24 hours to dry. The glue will dry up and you'll be left with something that looks much like what you started with, but set rock-solid. Brush or vacuum off anything which didn't stick, then apply more if you like or go on to the next bit. I usually do about a square foot at a time, but this phase goes very quickly and is extremely satisfying.
Here's the German layout about half-way through texturing. I did a bit less of this on Whitemarsh owing to all the long grass, and didn't take pictures of what I did do, but it's all there underneath.

Whitemarsh Yard
(NB: If you've read any of Dave Frary's scenery books, this should all be pretty familiar.)
Advanced: You can accelerate the process using Klear/Future as the adhesive, which has very low surface tension and may not need the wetting step. But you get through a lot of Klear and it's precious now that it's been discontinued.
More advanced: You can apply really fine texture by misting with neat or dilute isopropyl (rubbing) alcohol. You need a really well ventilated workspace to do this, please take care as it's both flammable and bad for you. The gluing part is the same.
GrassAfter that you get to do the long grass, if you're using it. For this I use long static grass fibres from Noch - I think these are better than anything else I've seen. They aren't blended so you can blend your own (important since most of the blends have red in for some reason - all you really want are greens and browns). The Woodland Scenics colours are good but the fibres have a glossy sheen to them which I don't like - the Noch ones are matt.
The technique I use is this one by Rick Reimer as I mentioned above:
http://www.youtube.com/watch?v=d2rOO1hN8_UI've tried glues other then Super 77, but it really is the best option - nothing else I tried was tacky enough to hold the fibres on end immediately. To do small areas you can mask off parts of the terrain with newspaper or painter's tape. Again, this needs really good ventilation as the glue is pretty evil stuff. If you don't have texture underneath then the step he describes where you sprinkle texture over the grass is pretty important.
You can add more texture using the dilute PVA/matt medium approach over the top of the grass, which is good for low-lying leaves and flowers.

Whitemarsh Yard
The other technique I used is from a Dutch group called the Modelspoorteam. In the picture above, the "hairy cigar" grass is the dry-looking stuff at the top of the banks. The richer grass down the banks uses grass mat (Noch again) cut up into random small pieces (postage stamp size or smaller) and put together like a jigsaw. Aileen's tacky glue is good for this, and you can also push the grass backing into the underlying terrain with tweezers or similar to lock it in place while the glue dries. It gives a more tussocky look, and you probably want to mix a couple of tones. I added in the odd patch of dead (brown) grass as well, especially down by the water where the roots will have died.
The water itself is acrylic pouring medium - this is self-levelling and provided you keep your layers thin - maybe 2-3mm - it dries clear and glossy. It can take a while to dry though - don't count on overnight, it could take a week to fully clear depending on the humidity level where you are.
PlantsI make these with all kinds of things - some coloured flock is pretty much essential, you should be able to get this in a model shop, and you'll want it for flower heads and petals. You can do without, but it's the little spots of colour that make the finished scene pretty.

Whitemarsh Yard
The cow parsley here is made by cutting short lengths of stranded hook up wire and twisting the strands together to shape the plant. I leave a bit of insulation on to hold the bottom of the stem together and make it easier to plant on the layout. The stalks are painted green, and the individual wire stands splayed out at the top to hold the flower heads. Dip the tops of the stalks in neat matte medium and then in white flock, and leave to dry. I use ink washes to tweak the colour once it's all dry.
To plant, poke a small hole in the terrain and push the base of the plant in. Glue is usually optional since the foam grabs well on its own. If you use angled tweezers you can usually make the hole and insert the plant in one go, which is more important than it sounds because it's easy to forget where the hole was in long grass...

Whitemarsh Yard
The rosebay willowherb (fireweed?) uses strands of dried grass sold as railway scenery. I paint matte medium on most of the length but not the ends, and tip the stalk in short static grass to make the leaves. Then dip the end in matte medium and pink scatter. Once dry, it helps to use an ink wash to tweak the colours and break up a big stand of the things. It sounds like a lot of work (and you need a lot of stalks) but it goes pretty quickly and doesn't require much though.

Whitemarsh Yard
The buddleia (purple plant on the embankment) is similar - like a version of the rosebay made with wire, and a bunch of them clumped together to make a bush. It helps to look at real plants and try and get everything so that it could be in season at the same time.
I'm sure talking about the bridge and concrete things would be teaching grandma to suck eggs, given the amazing quality of hard surface work on this forum, but if there are any specific questions feel free to ask. I don't claim any of this is particularly original or the right way to do things, but it works for me and is pretty quick to do.
HTH,
Will
[edit: have I done something bad with the images? The pop-ups don't seem to work whereas I thought they did before... I'm happy to fix if I'm doing it wrong.]