Ah, there's a few photos of that in the Dinorwic album. Technically it's not a bridge, it's what you're left with when the ground underneath a bit of railway track falls away 😮
I'm not at home at the minute, as soon as I get back I'll have a look in the Dinorwic book and give a proper description of where it is (up at the back of Twll Mawr) in terms of level and off which incline - and yes I know I should know...
my orders are to sit here and watch the world go by