...and old sleepers are very toxic, full of arsenic and all kinds of nastiness.
I'm usually the one moaning that this should be a DIY forum, but noseall is absolutely right in saying that it would be a mistake for anyone to retain soil with wood. Whether DIY or not, it would be a waste of time and money.
It's the quickest, cheapest and zero-skill way of doing the job, and it gets instant results so gets used on makeover telly programmes and by contractors who want the quickest way of making money and don't care about the long term. But it's complete rubbish.
Perhaps city types haven't been out in the wilds so don't know... wood actually grows, out of the ground, and is very capable of rotting back down into it. There's a good reason that woods aren't full of centuries worth of dead trees, it's because there's an army of insects, fungus and bacteria that find them delicious.
Preservatives make them toxic to life, so any that actually work have been banned, for very good reasons.