Locomotive building from scratch
Modellers build from scratch for one of two main reasons. One they just enjoy the challenge, and even though there are kits available, prefer to do it all themselves. Loco kits are not cheap to buy, so if you have the tools and the inclination, scratch building is a lot cheaper and a lot slower. The second reason is because the model they want is not yet available as a kit or ready to run. Then they have no choice.
To aid the scratch builder the Guild has a large number of drawings available.
There are various examples of scratch building published by the Guild in the Gazette and the forum.
Some guidance in locomotive construction is available as part of the guild manual.