How did you do the walls on Blackwood station and the quioning? When I last measured it the bevels were about 1" deep which is a little annoying in HO yet they're a pretty recognizable feature. Can't see from your photo whether you modeled them or not, but looks good from this distance.
The vast majority of my buildings use the cardboard sold as border material in picture framing. It is available from stationary shops and is usually around $6 for a large sheet (approximately 900 x 600 mm at a guess). If you are lucky the store may have sheets with slight damage that you might be able to buy cheap. I like them as they are 1.2mm thick so they are still relatively easy to cut. The bonus is this thickness is appriximately the same as a course of bricks. The other good thing is they are white on one side and coloured on the other. This means no need for paint before you clad over them and you can get away with not painting the interior either. The downside is I usually double the thickness around all doors and windows so i can get the depth of these right.
For the stonework I found some comercially available printed card that was a colour that I thought looked right. I didn't use the card but did scan it and print onto standard sheets of A4 paper which I simply glue to the structure. This way I don't add substantially to the thickness of the structure. The only problem here is the printer inks don't appear to be colorfast so beware if the building is going to be in a spot where it gets natural light as it may fade badly. The commercially available products don't appear to have this problem (or at least the fading is no where near as bad).
For the foundations I used the same card used for the wall construction laminated straight over the top of the existing structure and stone cladding. This makes the stone cladding of the building quite easy as it can overlap the bottom of the structure and be trimmed up when the glue dries rather than try to but it up to the foundation. If the foundation is concrete (like Blackwood) I will paint it using something like Floquil aged concrete. If it is brick I will use Metcalfe card. If the building also has brick quoins I will also cut them from Metcalfe card.
Now to the quoining on Blackwood, I have modelled it but don't have a good picture on file. From memory I had some good carboard that was around 0.4mm thick. When I say 'good' it looked similar to the card used in manila folders but it was more rigid. I know this is vague and probably not that helpful. An alternative may be to use cardboard from food packaging such as cereal boxes. I then marked out all the grooves required for all the quoins across the remains of the box. This gave me the best chance to get everything parrallel and aligned and ensured every corner of the building would look the same (if one was wrong they were all going to be wrong). Then I took to the cardboard with a triangular file to cut all the grooves across all the coins. Only then did I cut them out of the sheet. Patience (and maybe the use of a straight edge) may be required to file these lines straight.
You might be thinking that filing the cardboard is just going to make a mess and you would be partially correct. The cardboard does 'fuzz up' a bit when being hit with the file but this will only be the grooves being filed. From this point I would glue them (the quoins) to the building and paint (once the glue is dried). The trick is once the paint is dried you should be able to give the quoins a light file/sand (I use the triangular file again to try and keep the edges sharp) and any fuzz that might have occurred can be cleaned up as it is now hard from the paint. this may need to be repeated to get a really sharp result.
If uncertain, trial this before risking it on something important. The alternative is to use your scalpal on an angle to trim the grooves into the cardboard quoins but it is very easy to cut straight though cardboard this thin, and it can also produce inconsistent results.
I hope this is of some help.