| It is always cheapest to construct transit lines at ground
level, however, it is not always possible to do so. In areas where passenger loads are
high, it is often more economical to run rail lines in tunnels (subways) or elevated above
streets (els), than at ground level, and these types of lines are collectively known as
rapid transit. A number of els were constructed in US cities in the early 20th
century, however the effect of these on the surrounding areas was generally negative, and
most rapid transit lines built in recent years have been subways. |
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