This is from an EPA document

6. Are railroad ties safe for me to use for landscaping around my home?
There are no approved uses of creosote to treat wood for residential use. The Agency is aware that creosote-treated railroad ties are being used in the residential setting for landscape purposes and, in some instances, as a border around gardens. Such uses in residential settings are not intended uses of creosote and have not been considered in the preliminary risk assessment. If you do have creosote-treated wood in your yard, you are reminded to consult the handling precautions outlined above in this document.

I would never chance growing vegetables with any railroad ties, even if they are old. Free is not always the wisest.