Neelis, Jason (2007), "Passages to India: Śaka and Kuṣāṇa migrations in historical contexts", in Srinivasan, Doris (ed.), On the Cusp of an Era: Art in the Pre-Kuṣāṇa World, Routledge, pp. 55–94, ISBN978-90-04-15451-3 Quote: "Numerous passageways through the western frontiers of the Indian subcontinent in modern Pakistan and Afghanistan served as migration routes to South Asia from the Iranian plateau and the Central Asian steppes. Prehistoric and protohistoric exchanges across the Hindu Kush, Karakoram, and Himalaya ranges demonstrate earlier precedents for routes through the high mountain passes and river valleys in later historical periods. Typological similarities between Northern Neolithic sites in Kashmir and Swat and sites in the Tibetan plateau and northern China show that 'Mountain chains have often integrated rather than isolated peoples.' Ties between the trading post of Shortughai in Badakhshan (northeastern Afghanistan) and the lower Indus valley provide evidence for long-distance commercial networks and 'polymorphous relations' across the Hindu Kush until c. 1800 B.C.' The Bactria-Margiana Archaeological Complex (BMAC) may have functioned as a 'filter' for the introduction of Indo-Iranian languages to the northwestern Indian subcontinent, although routes and chronologies remain hypothetical. (page 55)"
Marshall, John (2013) [1960], A Guide to Taxila, Cambridge University Press, pp. 1–, ISBN978-1-107-61544-1 Quote: "Here also, in ancient days, was the meeting-place of three great trade-routes, one, from Hindustan and Eastern India, which was to become the 'royal highway' described by Megasthenes as running from Pataliputra to the north-west of the Maurya empire; the second from Western Asia through Bactria, Kapisi and Pushkalavati and so across the Indus at Ohind to Taxila; and the third from Kashmir and Central Asia by way of the Srinagar valley and Baramula to Mansehra and so down the Haripur valley. These three trade-routes, which carried the bulk of the traffic passing by land between India and Central and Western Asia, played an all-important part in the history of Taxila. (page 1)"
