Only time I saw saddle rails come off a plastic based saddle was when my youngest brother was rear ended by a car on his Gitane in the 80's and he crashed. We could not get the rails back on to the saddle base (Selle Italia Turbo) after that and had to throw it away. Luckily, the driver was nice enough and paid to replace it and the tacoed rear wheel on his bike.
Get a new one, I would not trust that saddle anymore anyway if you do get the rails back in.

Chombi