Weedwhacker said: "Weeds THRIVE in the cracks of the worst ones."
I don't think whether plants will grow in or around the ties is the issue -- more like what is the creosote (and whatever else; the RR uses some serious chemicals for weed control) doing in terms of contaminating the vegetables that we are consuming.
Absorption: Plants absorb less than 0.5% of the creosote available to them.
Impact: The impact of creosote on plants depends on the level of exposure.
Damage: Plants growing too close to creosote-treated lumber can be damaged.
Here are some other things to know about creosote:
Creosote can also be absorbed by animals, including voles, crickets, snails, pill bugs, and worms.
Creosote is readily biodegraded at low concentrations.
The health effects of contact with creosote depend on how much, for how long, and how you are exposed.
At that, I have used lot of creosote and only one RR tie ever still had a goodly amount of solid creosote in the railroad tie (which I found out when I sat on it on a warm day)
The harmful contents of creosote on a RR tie is so small, unless you chew on the wood it is not going to harm any thing.
IF, if it , and the ones you buy are decades old, for what ever reason still has enough active compound to be a problem, plant around will die or not grow at all.