Good comment. I suspect the B/C ratio would be miniscule!Very good comment.
The other crucial assumption being made is that the people commuting to Adelaide via the SE Freeway are largely destined for the city or locations adjacent to the line.
IMHO, this is highly unlikely; given the social demographic of Mt Barker, their destinations are far more likely to be scattered across the larger metro area, with probably a focus on the industrial and commercial areas in the suburbs S and SW of the city.
The people moving into the expanding parts of Mount Barker are not the typical hills commuters who live around Stirling etc, they are typically working/middle class people who have been priced out of the metro area.
New developments are springing up at Balhannah, Woodside etc with the same sort of residents.
It would be interesting to see the data if it exists, but I would expect that a good proportion of people riding the buses in from Mt Barker are then changing onto other buses after arriving in the city or at the bottom of the hill.