Shuz wrote:Steep slopes didn't stop them from building the freeway did it?
No, but steep slopes determined
where they built the freeway. To get from Crafers to the Adelaide Plains without detouring through the foothills, only two routes were suitable: Glen Osmond and Mount Osmond.
You seem to be concerned about the slope of the valley on either side of the river, of course that's going to be the case. I'm talking about the gradient up the river (moreso adjacent to it, which is favourable for a train line to navigate).
You've never actually been to Waterfall Gully, have you? The gradient is quite shallow in the suburb of Waterfall Gully (though the road and houses would prevent any railway being constructed). But it's not called
Waterfall Gully for nothing - at the southern end, there's a
near vertical drop where the big waterfall is! That alone would be enough to rule out a railway, but some of the gradients above the waterfall are also too steep.
It's just as much a non starter from a technical point of view as it is from an environmental point of view.