The SECTION CCCLVII of Shanti Parva of Mahabharat talks about a Brahmin who goes to meet a renowned Naga Padmanabha only to be told by his wife that he was out on an important errand which is quite interesting:
"The Brahmana said, 'O lady, I am sufficiently honoured by thee with the sweet words thou hast said unto me. The fatigue of my journey has also been dispelled. I desire, O blessed lady, to see thy excellent lord. This is my high object. This is the one object of my desire. It is for this reason that I have come today to the residence of the Naga, thy husband.'
"The wife of the Naga said, 'Reverend sir, my husband has gone to drag the car of Surya for a month. O learned Brahmana, he will be back in fifteen days, and will, without doubt show himself unto thee. I have thus told thee the reason of my husband's absence from home. Be that as it may, what else is there that I can do for thee? Tell me this!'
Furthermore when the Brahmin meets the Naga he addresses him thus:
"The Brahmana said, 'Thou goest away for dragging the one-wheeled car of Vivaswat according to thy turn. It behoveth thee to describe to me anything wonderful that thou mayst have noticed in those regions through which thou sojournest!'
I want to know if this story is detailed somewhere else. The second verse implies that it was a regular feature that was performed by Nagas as Padmanabha went there according to 'his turn'.
So my question is - Why would Surya's chariot need to be dragged when he has seven horses yoked to it and why was this Naga roped in for it?