What does "asleep at the switch" mean?

Also, asleep at the wheel. Inattentive, not doing one's job, as in At the critical moment the watchman was asleep at the switch and only called the fire department when it was too late. This term came from 19th-century American railroading, when it was the trainman's duty to switch cars from one track to another by means of manually operated levers. Should he fail to do so, trains could collide. It was later transferred to any lack of alertness. The wheel in the variant is a steering wheel; similarly disastrous results are implied.