红色很早以前就被用作火车信号，那时汽车还没出现，而铁路用机械臂板上升下降来代表前方是否有火车驶来。

Red has meant stop since long before cars existed, with train signals' use of red dating back to the days when mechanical arms lifted and lowered to indicate whether the rail ahead was clear.

youdao

红色很早以前就被用作火车信号，那时汽车还没出现，而铁路用机械臂板上升下降来代表前方是否有火车驶来。

Red has meant stop since long before cars existed, with train signals' use of red dating back to the days when mechanical arms lifted and lowered to indicate whether the rail ahead was clear.

youdao