Principal routes to the ocean lie around the north and the south (4,5)

'principal routes' is the definition.
'main lines' can be an answer for 'routes' (main line is a kind of route). I'm not certain of the 'principal' bit.

'to the ocean lie around the north and the south' is the wordplay.
'to the ocean' becomes 'main' ('main' can be a synonym of 'ocean').
'around' indicates putting letters inside.
'the north' becomes 'N' (abbreviation).
'and' is a charade indicator (letters next to each other).
'the south' becomes 's' (abbreviation).
'lie' placed around 'n' is 'line'.
'main'+'line'+'s'='MAIN LINES'

Can you help me to learn more?