OK, a quick how-to. It wasn't very high-tech, and owed a lot to serendipity.
To start out, this bit of track is an old piece of cheap flexi-track (not Peco) I found in the back of the garage - I bought it some years ago to strip the rail off and make 16mm scale Jubilee track panels, and this length was left over. The moulded chairs and fixings are quite large. Having glued it down I built up the ground on either side to sleeper level using pieces of 2mm card. I didn't have a lot of it, hence the gully at the side - another bit of serendipity.
The concrete slabs were made from 1mm card, cut to size then painted on top with a couple of coats of Heki Grundfarbe - Granit, which I bought in Germany more years ago than I care to remember - it's lasted very well. For the side slabs I chamfered the side butting up to the rail to fit over the moulded chairs, and for the slabs down the centre of the track I chamfered both sides.
A piece of the 2mm card with a piece of the centre slab, showing the Heki paint and the chamfering.
When I glued the centre slab down I weighted it in the middle so that the edges were really held down tightly against the chairs. The slabs were then brushed with MIG powdered pigment - the Industrial Grime and Concrete shades - to give a bit of variation in colouring.
Now for the cracks. At first I thought I could just draw some on with a very fine black pen, but that didn't look so good, so I pushed a sharp scalpel down into the card along the drawn crack, so that it made a cut in the card. The black ink gave the crack a bit of colour at the bottom, and more Industrial Grime was brushed over it to bring out the crack.
On the edges of the slabs the thinned card broke off a bit where it wasn't supported by the sleepers, and these bits were glued back at an angle to represent depressed fragments. At the end of the last slab pieces were cut off at angles, then glued down seperately with a bit of tilt and wide gap to represent a more broken up slab.
The weeds were made partly as Steve Bennett showed - Woodlands Scenics coarse foam soaked in PVA and laid in lines where required. However, I have just got some Silfor Grass Tufts, and they make it very easy to put little clumps of weed where required, as they are self-adhesive.
That's all there is to it! Not so quick as I thought, but I hope you get the gist.
By the way, I like the new Che Guevara pose for your avatar, Michael!