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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.

Generally, a railroad tie retaining wall consisting of a single line of ties should be between 3 and 4 feet tall—and the height often tops out at around 6 feet. You can go taller, but you may need additional footings and reinforcements, including a second layer of railroad ties to support the added weight of the soil it’s holding back.

A retaining wall can only function as a fence if it’s high enough on all sides to prevent someone from accidentally falling into the pool. However, most retaining walls retain dirt, which means their purpose is not related to safety. There would need to be nothing on the other side of the wall for it to be considered a fence, but in that case, it wouldn’t retain anything and would cease to be a retaining wall.  

Poured concrete is often the cheapest material for a retaining wall, costing around $6 to $8 per square foot. Other budget-friendly options include vinyl and landscaping timbers, which range from $10 to $15 per square foot; landscaping timbers are particularly well-suited for DIY projects. Untreated timber and railroad ties are a step up in price, starting at $15 to $25 per square foot. Natural stones can also be a cost-effective choice if you can source them from your own property, but they can be expensive to purchase. High-end materials like steel are typically the most expensive but offer superior durability and aesthetics.