I kind of doubt that, but I cannot tell because I cannot see the Capitaine Train booking engine without signing up. If they are proud of their site, why keep it hidden?

The other reason I doubt that it is any better than the SNCF site is because the SNCF site has improved quite a bit since they opened things up to travel agencies in 2009. So competition is a good thing, but the leader is still the leader. And the area where the SNCF has put in the most effort is mobile. A huge shift has taken place with phone and tablet bookings and the SNCF has many news apps which work very well and give us far more information than we ever had as travelers. They also opened up a real-time view of the entire network which is amazing. So if your train hasn't shown up yet, you can see exactly where it is.