I've been using Brooks saddles (7, including a Flyer, Imperial and 66) for years without snapping a rail. The weight has varied from 265-235, plus probably another 10 lbs in the bag at times. Ten of thousands of miles.
I'd have to summize that perhaps your seatpost is in some way causing undue strain. Is the saddle slid fore or aft?
This is Africa, 1943. War spits out its violence overhead and the sandy graveyard swallows it up. Her name is King Nine, B-25, medium bomber, Twelfth Air Force. On a hot, still morning she took off from Tunisia to bomb the southern tip of Italy. An errant piece of flak tore a hole in a wing tank and, like a wounded bird, this is where she landed, not to return on this day, or any other day.