Deakin is a remote locality on the Trans-Australian Railway, and it holds a particular distinction: it is the westernmost state's final siding before the boundary, the closest inhabited point to the line that separates Western Australia from South Australia. That line is the 129th meridian east - a longitude, an abstraction, a number on a chart that someone, at some point, had to find and fix onto the actual surface of the earth. Like its neighbours along the track, the siding carries a prime minister's name. It honours Alfred Deakin, who led the country three separate times between 1903 and 1910 and is remembered as one of the principal architects of Australian federation - a fitting namesake for a spot that marks where one part of the federation gives way to another.

Defining a border by longitude sounds simple until you try to do it in the field. A meridian is invisible; surveyors had to translate it into something you could stand beside. They did it in stages. First came the Deakin Pillar in 1921, a fixed reference point established by careful astronomical observation. From the Pillar, surveyors then determined the position of the Deakin Obelisk, completed in 1926 and set about 2.82 kilometres to the east. It is the obelisk, with its embedded copper plug, that does the real work: the boundary between the two states is the line taken through the centre of that plug. Two modest concrete markers, planted in the saltbush near the railway, between them resolve a question that maps can only gesture at - where, precisely, does one place become another?

There is something quietly remarkable about the effort involved. To place those markers, survey parties hauled instruments and supplies out onto one of the most featureless landscapes on the continent, took star sightings through the clear desert nights, and computed positions by hand to an accuracy that still defines a state line a century later. No mountain range marked the boundary, no river, no natural feature of any kind - only mathematics made physical in concrete and copper. The railway, running its famously straight course nearby, gave the surveyors a thread of civilisation to work from. Everything else they had to bring with them, including the very idea of a line where the ground itself suggested none.

Today Deakin is barely a place at all - a name on the timetable, a siding beside the track, a scattering of railway infrastructure on the plain. The Indian Pacific passes through without ceremony, and most travellers never register the moment the train slips from one state into the next. But the markers are still out there, the Pillar and the Obelisk standing in the saltbush as they have for a hundred years, quietly insisting on a distinction the landscape itself would never make. In a country this vast and this empty, it takes a deliberate human act - a copper plug, a careful line - to say: here, and not a metre further, the west comes to an end.