The answer was a simple system called " cut and cover."

问题的答案很简单，那就是：随挖随填技术。

These new lines, called tubes, were at varying depths, but usually about 25 meters deeper than the " cut and cover" lines.

这些新建的地铁线叫做地铁，它们的深度不同，但通常比随挖随填的隧道深 25 米。

By the late 1880s, the city had become too dense with buildings, sewers, and electric cables for the " cut and cover" technique, so a new system had to be devised.

十九世纪八十年代末，伦敦因楼房、污水管、和电缆外露而变得过于拥挤，原因是随挖随填技术，因此，人们需要设计新的设备。