I wouldn't have those railroad ties come in contact with the water. Creosote is in the right doses will kill your fish, as it is toxic.
What Health Risks does Creosote pose?
Because creosote is a general name for a mixture of different chemicals, and limited research on health effects has been completed, it is difficult to quantify the risks. Creosote has been classified by the Environmental Protection Agency (EPA) as a probable carcinogen. Studies have shown increased risk of cancer and respiratory problems in plant workers repeatedly exposed to creosote. Exposure to creosote on the skin has been shown to cause rash and irritation and in an extreme case, cancer. Direct skin contact with creosote poses the most likely health risks to people outside industrial applications.
Information from here: http://waterquality.montana.edu/docs/wqfaqarchives/wq_faq_3.shtml
What Health Risks does Creosote pose?
Because creosote is a general name for a mixture of different chemicals, and limited research on health effects has been completed, it is difficult to quantify the risks. Creosote has been classified by the Environmental Protection Agency (EPA) as a probable carcinogen. Studies have shown increased risk of cancer and respiratory problems in plant workers repeatedly exposed to creosote. Exposure to creosote on the skin has been shown to cause rash and irritation and in an extreme case, cancer. Direct skin contact with creosote poses the most likely health risks to people outside industrial applications.
Information from here: http://waterquality.montana.edu/docs/wqfaqarchives/wq_faq_3.shtml