I'm new by most folks' standards, but it sounds to me like you're talking about the Pipeline Hole. I've only seen it from a distance since, from what I understand, poor behavior by a handful of people forced the landowner to cut off access completely. It's a green bridge with a big pipe running through it, can't speak for a cart though. It's just downstream from Dodge Park, but I wouldn't recommend risking it. I went for a peek one day and was greeted by a Clackamas County Sheriff's Deputy who was on his way to hand out some paperwork to some other guys who tried trespassing to fish it.
But he DID tell me there's a legal way to access miles of river downstream and gave me directions, but I won't try to repeat them since it just got me wandering down some old game trails a few miles back in the woods.