Portal tracks can be purchased in your Building menu, Settlements tab after you unlock the research in the tech tree. They are built like our normal city roads, one square at a time. To build each 1 x 1 track you will need 40,000 coin and 10,000 supplies.
Portal tracks are a special kind of street that connects Dwarven production facilities (granite and copper mines) with the Portal where the Goods are stored.