A cable thin enough to go through the saddle rails will be thin enough to cut quite easily.

What I do is use a thick cable with a loop on each end. The cable loops through the front wheel and through itself, then through the front fork, and the other loop goes into the D-lock that secures the frame and the back wheel. Then I use a padlock to lock the saddle rail to the cable. The saddle rail is now probably the weakest point by a fair margin, and of course stealing the saddle by breaking the rail renders the saddle useless.