The two earlier routes of the Eastern Railway followed closely the line of two creeks - the first route followed Nyaania Creek which is part of the Helena River catchment, while the second followed Jane Brook which joins the Swan River at Middle Swan.
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The unit's first major operation was constructing a suspension bridge over the Helena River at Guildford in Western Australia.
The river valley is the centre of the catchment of the Helena River and extends from the edge of the Darling Scarp where Boya, Gooseberry Hill and Helena Valley define the 'mouth' of the valley that opens to the Swan Coastal Plain, east and south east past Mundaring Weir and Lake C.Y. O'Connor to its origins in the region of Mount Dale.