The amount of creosote and related materials leaching out of old railway sleepers will be minimal in normal "domestic" usage, such as around a vegetable bed. The primary routes of human exposure to creosote in use are dermal and inhalation. I know that Morgan owners are a breed apart, but probably none of us rub ourselves repeatedly on old railway sleepers, or indeed sniff them. Thus re-use ticks all the boxes for me.
I don't think creosote can be that bad for you, I can still buy it in 25litre barrels from my local farm supplies place. There really isn't anything better for preserving wood.
It's supposedly carcinogenic, apparently based on a study involving weeks of constant contact with unprotected skin in rats, but I can't see that having a few railway sleepers about the place is going to cause an issue.
