i am making a raised bed with railway sleepers - what do I put in the bottom - i want it organic so do I fill it with compost?

Be careful of using railway sleepers in a raised bed for veggies, they are soaked in creosote and coated in tar... not conductive to good growing for veggies.

May I ask on what you are putting the sleepers? If on hard ground like tarmac or concrete I'd put some gravel in first to aid drainage. If on natural then an old piece of hessian backed carpet to ensure nothing untowards grows up from below, then with your compost and sharpe sand. It may be an idea to line the inside of the sleepers with some durable plastic to aviod the wood rotting, it would also avoid any 'nasties' in the sleepers effecting your soil.

Akela65 beat me to it, but I would use a plastic liner to cover the underneath bit and sides of the sleepers. This prevents any "nasties" getting into your "organic" veggies!

Not sure about the carpet. If you want to grow veggies that need deep roots, the carpet will prevent this. If you do want to use something, you could always use a builders merchants black membrane, which allows water to penetrate but nothing to grow up thro' it.

I personally would use a mix of soil and compost. Depends on how rich the soil is tho' too. If it's really clay or poor soil, just use compost. You could buy in some "top soil" to mix with compost but be aware there will be some weeds. Don't buy sterilised soil tho', all the important bio. and organisms will be missing. (not 100% on the science bits, lol!)