Soil test show that those creosote railroad ties can be broken down by fungi, for sure. I would have a tendency to parent crop with with the red garden giant mushroom and plant some non edible companion plants like lupine, straw flowers and maybe some flower bulbs ( good as a secondary income or area beautification) as well as a good nitrogen fixing cover crop for when you detoxify the hugel beds. At our place we use vetch and clover as well as a mix of alfalfa and barley to mix up the root depths. Don't forget to get a decent inoculant for your seed when you plant it so you can enjoy all the nutrient fixing benefits. Our hugel mounds are in their third year and a rocking out number crops of potatoes, garlic, corn/beans/squash, tomatoes, all kinds of medicinal herbs and spices as well as some fruit trees planted down hill to block the wind an retain the back fill. Happy hugelkulture and have fun.