The Grand Trunk Road links the eastern and western regions of the subcontinent, running from Peshawar, across north India, into Bengal. Built several centuries ago, it is one of South Asia's oldest and longest major roads.
By hailing it as the ‘backbone of all Hind’, Kim is referring to India by the shortened form of the name Hindustan.
In Kipling's time, a bride being transported in a procession alongside her groom would almost certainly have been on her way to the family house of her husband. This is not a specific Hindu tradition, but is a custom across many of the various religions practised in India.
Indian weddings are known the world over for their colour and exuberance. Filled with ritual and celebration, they can last several days. While the bride in Kim will most likely have had an arranged marriage, today an increasing number of couples are opting for love marriages.