Posted: 13/05/2016 at 13:40
Just to add to your confusion:
Personally, I wouldn't use old railway sleepers unless you've already bought them. Can't remember exactly what they're treated with (bitumen?) but they can ooze a tar like substance when they get hot. It can make a right mess of clothes and (presumably) can leech into the soil.
Mine were built using new pressure treated softwood sleepers. 2.4m long - 20cm square cross section.
Highly durable and no leeching other than the odd bit of resin. Mine are 2' high so nice to sit on
We stripped the grass and turned the turf upside down in the bottom of the bed. Then a mix of soil / spent mushroom compost and a bit of well rotted farmyard manure. No hardcore at all.
Remember not to make your mix too rich - seedlings and a lot of root veg (carrots etc) don't like lots of manure. We didn't have a great harvest the first season but by the second year everything had rotted down / mixed together well and we had an excellent crop.
Then the voles moved in.....