The massive, epensive and inflexible nature of elevated rail structures and systems render them the least adaptable on all transportation system types. While subway tunnels could be fairly simply refit for installation of monorails, the massive elevated concrete structures required to support elevated rail systems would be greatly overbuil to support monorails, yet not adaptable for street, highway or elevated busways.
With the adaptability of elevated rail systems so severely limited, rigorous consideration of the long-term serviceability of elevated rail systems should be an important consideration in the initial evaluation of transportation modes and technology for specific service objectives. Current trends in modernization of transportation technology should be considered relative to the limited adaptability of elevated rail systems and facilities. Inability to identify practical adaptability of elevated rail systems to improvements in rail or other transportation systems and technologies should shorten the projected lifespan of elevated rail systems, and require development of plans for demolition and removal of structures and facilities at some time in the future. The costs and logistics of such demolition and removal of facilities should be included in current planning and projected project costs.