Other tools that may be considered are a magnifying glass on stand (for those of us less blessed in the vision area), small drills, small files, fine sandpaper (also useful when glued to flat or rounded sticks), superglue.
Sometimes it is best to sit back and visualise what the final shape should be before scoring or cutting out the part.
Use a sharp knife with a steel ruler to cut straight lines. I now use contact adhesive for nearly all construction.
In many places the card needs to be folded along a line. Score the card on the outside of the fold. This may sometimes mean scoring on the back of the card. In order to do this accurately it may help to prick through the card with a pin to leave a mark on the reverse side. In most cases I give minimal indication of where a fold line is but it should normally be fairly obvious.
Where a score line extends into a cut line, score the whole line first then cut through where required afterwards. This will keep everything lined up properly.
Where card needs to be curved roll a rod or dowel over the card whilst on a cushioned surface, gradually increasing the curvature. Items such as boilers can benefit from being permanently formed around a solid cylinder. Find a suitable wooden rod and wrap a strip of paper around it to bring it up to the correct diameter then sand down the “step”.
It is almost impossible to curve a piece of card in two directions as it can only be achieved by stretching the card locally. In the past the use of a warmed spoon has been suggested. I’m not convinced that even this works.
To cut external or internal curves it is possible to make several straight cuts and finish with sandpaper. If you have a punch of the correct size it may be used for circular items such as cab windows and particularly for buffer heads, which will then assume a dished profile. Small holes can be drilled but it is difficult to avoid raising a burr at the edges of the hole.
The most useful glues are contact adhesives and superglues. Apply them indirectly using a cocktail stick or similar to reduce excess glue.Try to use as little glue as possible to avoid excess on the surface and also to avoid wetting and swelling the card. Try to use paint sparingly for the same reason.
Small features can be made from wood or plastic and wood can be also used to fill enclosed spaces such as the loco chassis. Useful sources of small pieces of wood are cocktail sticks (particularly part of the tapered ends for buffers), matchsticks and ice-lolly sticks. The profiles for some parts are provided so that if you have the means items such as the dome can be turned from the solid.
Quite often a small domed top feature is required such as GWR tank locomotive tank ventilators. A quick way to make these is to hold the end of a plastic rod close to a heat source and the plastic will melt back into a dome. I use a candle flame but there are safer ways of doing this. The rod can then be cut to length (this method can also be used to produce fake rivets for larger scale models, drill a hole and put the “rivet” stem through).
For fake square or hexagonal nuts form a square or hexagonal rod and cut slices from it.
Openings can be glazed using such products as a PVA glue that dries to form a clear film.
A coal wagonload can be represented by a black painted piece of coarse sandpaper, perhaps with extra sand added to give a central hump.
Don’t forget that many wagon loads were protected by tarpaulin sheets, which were usually lettered with the railway company initials and numbered.