The pertinent points of this painting: repetition of shape, vertical lines, horizontal lines, numbering/lettering, ellipses and in this particular exercise a very clear one point perspective.  

The watercolour techniques to use are: wet on wet, glazing (for the steam), working in glazes and working light to dark. The subtle difference in glazing and working in glazes: glazing is when you put a wash of colour over a dry paint to make it look misty, foggy or smoky. Subsequent glazes are when you work in light washes to start and when the washes are dry you put stronger tone paint over the dry paint. I sometimes work at least six glazes, from the lightest to the darkest tones of a colour. 

Brushstrokes to use: pointillism/stipple (all the trees in the background and the shingle in the foreground), chisel brush for larger brushstrokes, a large round headed brush to apply the glaze for the steam, circular brushstroke. 

A Good Idea is to either go train spotting at a steam train tourist spot or study photographs of steam trains so you get a general and then more specific idea of their overall structure. Make some preliminary sketches too.

Once completed you will need a variety of colours, I used reds, yellows, blues, greens, flesh tint, chromatic blacks, white, brown, burnt sienna, burnt umber, a mix of my own browns, purple I think that was all! I painted the front of the train first and then put on the washes for the tops of the carriages. I then painted the foreground yellow for base colour of the grass and used flesh tint for the base of the shingles. I painted in the sky, wet on wet, a nice bright blue. I then painted in the trees. This was the longest task in this painting. I used a wide variety of greens, working from yellow green through to a black looking green, that is a dark green with a dark brown added.  

I then went back to painting the train, my reason for completing the background was it gave me insight into how dark the tones of the train should be in contrast. I also used a watercolour pencil for the smaller shapes of the windows, more easy to manipulate for the smaller areas. 

The light source is clearly from the left hand side, so there are some lovely shadows cast by the small wall in the foreground, the train, and some of the parts on the front of the train.  

I then applied the steam/smoke. I used a large round headed brush, a white gouache, a minute amount of light blue was added to some of the gouache. I used a circular brushstroke to create the effect of moving smoke, using a quick brush movement so as not to disturb the paint under the glaze. 

The numbering on the front of the train: draw in the shapes carefully of the numbers. You can either use the negative space around the numbers to make their shape or paint in the shapes of the numbers using gouache, which is the method I chose.

I took lots of time refining each part of this painting. The overall painting took 11 hours to complete.