Possibly, yes, but it depends on whether the wood has been pressure treated or tanalized, and precisely what compounds were used to carry out the treatment. Most modern treatments are not toxic, but new sleepers are often treated with creosote, and should not be used, though old ones should be fine. The biggest problem using decking planks, especially if its softwood, will be the rate of decay, which will be quite fast when in contact with soil.

Have had raised veg beds made from decking planks for five years now. They will be fine if you line the inside of the planks with strong black plastic before you fill them - so no danger of contamination and minimal rotting.

Here in the States, most modern treated wood has been pressurized with a solution of arsenic salts--still very toxic. Is treated wood different in the UK, Bamboo?

Yes, luckily, Tugbrethil - most of the noxious preservatives were withdrawn and banned at least 10 years ago. We have the EU to thank for that.

Ah! Wise of you-all! I'm all in favor of civil liberties, but sometimes the laissez-faire of the U.S. system winds up biting us where it hurts.

Yes, not least regarding GM crops... but frankly, I suspect the UK would be the same as the US with the laissez faire attitude, its Europe that's been keen to abolish certain substances.