I have to diaagree with some here regarding the use of Rail Europe in fact I cannot think any reason to use Rail Rurope ( except they have all the railways concentrated on a single site) or they are running a rare special.
Not insignificant are the delivery charges to have the tickets delivered in the US
For SNCF, if you cannot find the tickets on the SNCF website I suggest calling their English speaking line and you might find be able to find tickets that are partially discounted by buying 30 or 45 days out, tickets fron two or groups travelling together etc and other specials not available on RE.
Added to the above the convenience of exchange/refunds at any station.
mike