The weight of the train has to be dealt with. They either have to have a serious under-bodice and boning to bear the weight or a very sheer train. You can't just sew hooks and eyes onto the shoulders of most existing gowns and have the dress not be pulled backwards or ripped.

The way court trains were done in Napolean's court might work: the train was sewn to an extremely short "jacket" that was nothing more than the shoulder part of a jacket with straps that fastened around under the bust and around the arms. It's like a harness with train attached. Look up Empress Josephine and that era in costume books.

Watteau trains are not that difficult. I am altering a Maggie Sattero gown right now that has one. It is made of Gossamer chiffon. You make it as long as you want. Round the train end with a rolled hem. The other end that fastens to the back of the halter is gathered to about 3-4". This particular one has snaps attaching it to the alter and then an applique ovr the top to cover the snaps. Then it just drapes nicely. It is beautiful.