Cheers Jim -
I will need to look in my railway library for the answer, but I would say the original Act of Parliament to build the railway, by authorising a station at Dartmouth, put a legal obligation on the company to build one, and the ferry terminal was the result. There were lots of similar shenannigans in the early railway days.
Reminds me of a tale I heard from an old mate who worked on the NSW railways. In far west NSW, on the line to Broken Hill, there is a station named Ivanhoe, serving the town of that name. This is country where it is a forty minute drive to your front gate, and another hour-and-a-half to get to the shops. Sometime in the 1950s, in century heat, the weekly train arrived and out got a dapper little bloke in a brown paper suit and polkadot bow-tie. As the guard handed down his suitcase of samples, the salesman asked "Where's the town?"
"It's over that way"
Sure enough, just visible through the heat haze and shimmering mirages, a pub and a few houses could be seen at a couple of miles' distance across the plain.
"Geez, why did they build the station so far from the town?"
"Well, cobber, they probably wanted to have it near the railway line"...
