Ball signal is the most common signal on the early railways, the ball(Red) gave rise to the term "highball". When it was raised, it was safe to proceed. This was later reversed.
<a href="//commons.wikimedia.org/wiki/User:Charlie_fong" title="User:Charlie fong">Charlie fong</a>
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<span class="int-own-work" lang="en">Own work</span>