Have you ever looked at the railroad ties that are available at my local Home Depot. They are conveniently right next to the commercially-produced landscaping timber. But I love the look and feel of railroad ties plus the lack of hard corners.
Yet the EPA has reasons to consider not using them.
Number 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.
It's a coal-tar product, and coal-tar produces tumors in rats and mice.
Modern treatments for residential-use landscaping timbers no longer invariably use arsenic or other potentially carcinogenic chemicals.
This is voluntary compliance on the part of manufacturers, who aren't stupid, and who realized during the CCA dustup about 10 years ago that consumers aren't going to buy poisonous lumber to build their decks and swingsets.
So if you're talking about ordinary landscaping timbers that you buy at the lumberyard or Home Depot, check for the sticker or inked stamp that says how it's been treated and that it's safe for your kids to climb on.
If you're talking about actual used railroad ties, I wouldn't, because, carcinogens aside, the creosote is messy, plus splinters. Check with your local university extension office for recommendations for pressure-treated landscape timbers that will last well in your climate and with your soils.
Properly pressure-treated purpose-built landscape timbers (i.e. not simply plain lumber that you're using) should last at least 10 to 20 years, depending on how wet they are all the time.
Make sure they're designated for ground contact.
You stack landscape timbers by drilling holes and spiking them. Overview.
When you say your play area is 30 feet square, do you mean it's 30 feet on a side, or is it 3 feet by 10 feet?
If it's 30 feet on a side, that's 900 square feet, not "30 feet square", which makes a difference if you're ordering mulch or lumber.