Unfortunately, not ideal!
They tend to have bits of ballast embedded in them and those chips don't do any saw blade any good and can get flung out almost as fast as a bullet. Not to mention what the centre portion between the rails can get showered with (unless you get either an unlaid one or one from a station [in theiry...]). I have a feeling they're treated with some fairly noxious stuff to stop them rotting too - and a splinter from one will be NOT fun. The gunk also makes it hard to paint them - the stain comes through.
You could try your local sawmill (check yellow pages) and ask them if they've got any big (often cracked) blocks.
[ignore! Seen your location!] Finally, if you're anywhere near Exeter and can collect, I MAY be able to find a couple of 18" (or so) cubes unless Dad's found a use for his! (Which came from a sawmill)[/ignore...]