Preheat the oven to 180°C/Gas 4
Butter a 9"x 13"/23cm x 33cm Swiss roll tin, or something of similar dimensions and line the base with baking parchment.
Blend the cocoa and water and allow to cool slightly. Add the other ingredients into a large bowl and mix. Beat together with an electric whisk at a fairly high speed for one minute until the mixture is smooth.
Place the tin into the centre of the oven. Bake for 25 - 30 minutes, insert a cocktail stick or metal skewer to check if the cake is cooked evenly.
Let the tin cool on a wire rack and wait 5 minutes before removing the cake from the tin and let it completely cool on the wire rack.
Cut the slab cake into 12 squares by cutting a row of 4 by 3 squares.
Place the cakes on a large board making various trucks using one pice of cake, and carriages by stacking 2 together using a little buttercream.
The engine was made by sculpting a cylinder on the second layer and cutting two squares in half and stacking them for the drivers cabin. Secure these together using buttercream.
For a simple buttercream icing mix butter with the icing sugar. Continue to mix until smooth adding the milk to loosen the buttercream. Split into 4 bowls and colour them bright colours such as red, blue, green and yellow using food colouring.
Using a small palette knife carefully cover each truck and carriage with a layer of brightly coloured buttercream, this will be much easier if you chill the sponge pieces and gently wipe off any crumbs first. If the buttercream sticks to the knife, heat the knife in a glass of hot water, wipe quickly and smooth the buttercream, repeat if necessary.
Now have fun covering the cakes with various sprinkles etc, using footballs for wheels and animal heads in the windows of the carriages!