Answer: Some forms of creosote and/or components of creosote are known human carcinogens, so contact with the compound is not recommended. Aged ties are less likely to have volatile components that can irritate the lungs and mucous membranes, but you should wear gloves and long-sleeved shirts to prevent skin contact at any rate. Here's a link to a site that provides detailed health effects as well as a good overview of all the chemical compounds known as creosote. And according to this document, some of the components of coal tar creosote, which is used for railroad ties, can leach into soil and groundwater, where they take a long time to biodegrade. Nevertheless, it's still a very commonly-used preservative. I hope this helps you make a decision!
Creosote Chemical Backgrounder: http://www.nsc.org/ehc/ew/chems/creosote.htm
Q&A Library Searching Tips