Scyscrapers.
Streetcars made growth of suburbs possible (otherwise people wouldn't settle so far from center, where they were working). Steel and glass had a similar effect on scyscrapers - it made their construction possible.
Streetcars in the 1800s enabled the growth of suburbs by allowing people to live further from city centers, while glass and steel were crucial for constructing skyscrapers. Both innovations transformed urban landscapes by facilitating expansion and creating new architectural possibilities. Therefore, the missing part of the analogy is 'skyscrapers.'
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