Soil Safety of Vegetable Bed with Rail Road Tie Sides?

We are transforming a landscaped garden at my daughter's school into a vegetable garden. The garden had raised beds made from creosote impregnated RR ties that have been in place for thirty years. We are concerned about soil contamination.

We have removed 6 inches of soil from the top and sides of the beds that were in direct contact with the ties. Do you feel this is adequate or do we need to remove all of the soil from the beds?

Hardiness Zone: 11

By karen from Oakland, CA