That's a coincidence... I had a level 2 bridge for 14308 exactly and I thought it was clean at first, but when I tested it a few more times it fell to bits :-O
Did you give it time to settle before you ran the train over, or run it over straight away?
(to do that I hit pause, reset, run train then hit pause again to get things moving)
Crap, this is wierd, I just ran the exact same design on my pc at home (P3-750) and on the 4th and final run a single link broke... but at my PC at work (P2-400) it cleared all 4 passes without a single break! I wonder if the CPU speed affects the velocity of the train which in turn affects the stresses on the bridge.