Building using recycled materials certainly makes ecological sense, however, they need to be understood so as to be used most effectively. Used railway sleepers are often used in new building work, but they are usually not treated and can also be a problem when attached to a building. It's best to avoid using any recycled susceptible timber if it comes into contact with the ground.
A retaining wall/landscaping timber attached to a building, causing a potential breaching point for termites.
These timbers will negatively impact both a physical and chemical barrier.