Answer: In my opinion, railroad ties are not a good thing to use in the vegetable or herb garden, especially since there are alternatives available such as the new recycled plastic "wood" plus the old standby's: rock, brick, cinder block, redwood, cedar, cedar shakes on end, and plain sharp earthed edges to the raised beds.
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 still 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!
The National Safety Council site at http://www.crossroads.nsc.org/chemicals.cfm
allows you to search for creosote.
Q&A Library Searching Tips