Treating railway sleepers

We used new railway sleepers (which we assume were weather treated in some way) to create retaining walls about 5 years ago. Where plants trailed over the top of them, and created more moisture, they have started to disintegrate. We need to seal them on the end with some form of preservative. Everything we have seen at the DIY store has colour/stain, which we don't want. Would Linseed Oil work? If it is pure Linseed Oil, with no other nasties added, would this be OK to use near plants/veg? Any other suggestions welcome.
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I came across this stuff and is what I've used.
It's clear and has the consistency of thick water... but was easy to apply.
It's water soluble and I don't think there are any ingredients that would harm your plants once dry.
There was no apparent residue from drips on the paving
Wisdom is not putting it in a fruit salad.
I use a conventional coloured paint treatment on my wood, but I've got the above varnish, and I don't think it has anything unpleasant in it. Can't remember why I bought it though
Wisdom is not putting it in a fruit salad.
Don't think my pond's quite big enough Pete