We used pressure treated 4x4s, 16 feet long I think as one of our gardens is along the fence line. We stacked them in some areas where the contour of the land changes. We have had no problems with them, and I think they are easier to work with than railroad ties. They look good too.
Just a guess, but I think the chemicals they treat the ties with are not very good for a garden - some I've seen actually leaked tar for years!
You could also use that plastic edging stuff if you don't need to build it up too high.
Simon

I'm not sure where you're located but wood treated with CCA's are now prohibited in the states. This affects wood that has been treated with arsenic not the newer wood that's been treated with a new copper-based preservative. This change went into effect on 1/1/04.

CBS News article about pressure treated wood

Here is a link that might be useful: EPA Factsheet on pressure treated wood

I have 4 raised beds in my yard, made from wood boards. They were here when we bought our house so I don't know how old they are, but 2 are older & starting to rot. This year, I want to put cinder blocks around them. They would be higher than the existing wood, would last longer, and cinder blocks are fairly cheap. I would like to plant marigolds in the holes of the blocks. (Do marigolds really keep critters away from the veggies?)
Since this is just one of my projects planned for this spring, so we'll see how far I get with it.

Here is a link that might be useful: Vegetable gardens

Yes, pass on the railroad ties. One landscaper friend calls them something like "tacky, tarry and toxic". They make your garden smell like a wharf; the creosote coating washes and flakes off into the soil, adheres to anything placed onto or brushed against it.

I live in FL and I'm trying to get the rest of the family (most in SW PA) to relocate Mom's blueberry patch for Mother's Day. I grow them down here and she used to have a nice patch of wild ones on her property that she enjoyed. They've disappeared over the years. I've found the blue berries I'm going to order for her. With the one in a pot she already has that will make four. Each plant needs four-six feet of space. Since she's 4'7" it can't be too wide. If they're closer to the house it will be better for her. Some companion edibles would be nice--any ideas?