“May cross over at different points in time and they may proceed in parallel.” (Brannen & Nilsen)
“Rails for a tramcar.” (Pearsall, 2003).
Used as a metaphor to convey the trajectories of different careers in work, parenthood etc. since careers intersect at different points in the life course. (Elder,1978).

Brannen, J. & Nilsen, A. (2003, January 28). Adulthood: Changing Concepts and Definitions, A Sloan Work and Family Encyclopedia Entry. Chestnut Hill, MA: Boston College. Social Networks. (2003). In Judy Pearsall, (Ed.), The concise Oxford dictionary in English dictionaries and thesauruses. UK: Oxford University Press, Inc. Internet (13 June 2003). Elder, G. (1978). Family history and the life course. In T. K. Hareven (Ed.), Transitions: the family and the life course in historical perspective. New York: Academic Press.