The wood used for railroad ties iss treated with creosote. Creosote is a known carcinogen. However, the carcinogenic effect is from handling, inhaling or ingesting the creosote. Old, weathered railroad ties present an extremely low level of risk to you. The creosote is old, weathered, will not dissolve and will not leach from the wood at this point in time. The creosote will help slow down the process of wood decay. In my opinion, and I have discussed this topic with an environmental toxicologist at the University of California at Davis, the railroad ties present no risk to you as a gardener and will not harm any of the plants or vegetables you grow in or near the railroad ties. My colleague in Davis agrees.