Old steel elevated tracks like the L are kinda crummy. You have to deal with weather both on the tracks and the stations, they darken whatever is below them, are incredibly loud, and wear out relatively quickly. Derailments can be catastrophic because trains fall from a great height (as happened in the Loop in 1977). Using concrete solves the noise problems, but requires more space and money.
Concrete adds a lot more bulk and makes them visually oppressive. I’ve always hated how they did the elevated Metro lines (hulking concrete structures high in the air, requiring climbing up long escalators with multiple mezzanines), versus the L lines which are organically integrated into the neighborhood.
