Railroad ties can be good for retaining walls, but it depends on the area. They have a desirable rustic look and are durable. The problem is they’re treated with creosote, a preservative the EPA says poses a risk to humans and wildlife. Over time, the preservative seeps out of the wood and into the ground. Many areas have banned the use of creosote-treated wood on residential properties. If your area does allow it, never use railroad ties near an edible garden or drinking water source. Instead, use wood that’s treated with an EPA-approved preservative or a wood-alternative composite material.

Small retaining walls still require a foundation of some kind, typically a concrete footing. This helps to ensure the wall is structurally sound and prevents the ground from shifting over time, especially during those winter freeze/thaw cycles. The footing helps spread the weight of the wall and the load it carries over a larger area of the ground, thus assisting with stability. This is true of small retaining walls, larger rock garden walls, and the like. 

Through proper drainage solutions and compacting the foundation soil.

Failure to compact the soil during construction is the most common cause of leaning retaining walls. In addition, poor drainage can lead to excessive water pressure behind a retaining wall, pushing against it until it begins to move forward away from the soil it is holding back.