短语asleep at the switch可以按字面的意思解释:该给火车扳道时,扳道夫却睡着了,结果自然是火车出大事故。不过,这种情况总不会多,而这个短语被更多用于比喻一个人坐失良机,如:
A deer came in sight, but he was asleep at the switch and did not fire a single shot. (一头鹿出现了,但他坐失良机,没有开一枪。)
这句话并非指他真睡着了。如果指真睡着了,就不必在asleep后加上at the switch.