I'm not sure about giving them a longer life but I agree using polythene is a bad idea. Sleepers should last decades if they've had some sort of rot prevention treatment.

Most railway sleepers have been soaked with coal tar or--in the case of newer ones--pressure-treated with arsenic compounds. Bad for their lifespan or not, they need to be covered with polyethylene sheet, preferably a heavy gauge, everywhere they touch the soil, if you are planning on growing anything edible in that raised bed. I wouldn't expect any membrane to help keep them from rotting, though if they are very irregular, weed barrier is a way of keeping soil from oozing out of the cracks.

Tugbrethil is bang on but it doesn't matter what you try to grow in a bed if the sleepers have been treated with coal tar or arsenic. They will kill the plants unless completely isolated with something impermeable. (I speak from bitter experience from my callow youth.)

Google railway sleepers (untreated), buy 2 8ft. and cut them in half.

Personally I've never understood the need for railway sleepers. I'd use treated 6 by 2 inch planks similar to those used on scaffolding on building sites or use brick or block work . All of these could be disguised if required by planting around with evergreen plants.

You can also buy plastic ones. My next door neighbour bought one earlier this year. Here's a photo I took of it with details of the size and cost.

http://www.flickr.com/photos/31559373@N00/3554888794/

After a bit of googling it looks like railway sleepers cost £20 to £40 each depending on whether they're new or old and what they're made from. Further Googling shows scaffolding planks to vary in cost from about £4 to 10 depending on size and whether they're new or old.

Once fitted they'll look hardly any different from a sleeper and will take up much less soil space and are much easier to transport/handle..

Its no longer possible to buy old railway sleepers in the Uk.

I've just checked Google again and there is at least one UK site selling used railway sleepers( they do say they are recycled from European railways)

Doing a bit more research it looks at though certain sleepers with certain creosotes on them were banned.

Yes I should have said theta sales of old railway sleepers in the UK were banned about 8 years ago because of the creosote content.

As Anchorman says bricks or blocks can be used. I have used these for many years, they are so easy to move from one place to another as I do each year. You can get reclaimed ones from a yard that is in your area where many agregates, bricks, soils etc. are recycled.

I used scaffolding planks to make a whole row of raised veg beds, (see my photo's page 2,3 & 4 I think). Cost £4 each, used the full length & one cut in half for the ends. Treated in environ friendly stuff, perfect height to give a decent growing depth.

Excellent work Bampy. Your raised beds look great and very producive.

cheers Anchor, they are doing well for new beds. I simply layed the frames straight onto the grass then turned the turf over with a spade grass side down & then barrowed tons of fresh stable manure, top soil & garden compost on top, then a top-up when it settled as expected. So much produce this year is was amazing.
I treated myself to a £60 electric tiller specifically for the raised beds ( there is a lot of area to cover all in all ) & what a back-saving bonus it was mixing all the ingredients together plus it now mix's fresh compost in in minuts rather hours.

What did you use to treat the timbers of the beds?