Rob,
Here are some pictures of rail guns at Rosebud this year:
Rail guns in the context of Benchrest shooting fit into the unlimited class of competion. As far as I know the only club holding unlimited class shoots in Canada is Rosebud, just West of Calgary. Unlimited class is a group shoot, shooting either 5 or 10 shot groups.
Pretty much anything goes in terms of design or construction, except they can't be attached to the bench. In most cases they are a free recoiling rifle, that rides on a set of rails or wheels or bearings that allow the rifle to recoil and then be pushed back into battery for the next shot. They hold perfect point of aim between shots, and there will be some knobs for adjusting aim or holding for the wind. Like all Benchrest matches it is essentially a wind reading contest, except in this case you can use pretty much any rifle you want. ie no weight limit.