Lost Track of Time - the Idiom

My ESL-teaching friend slipped during class, and said, "I lost track of time." One of her students rapidly looked up each word in the dictionary, and then took out his cell phone and shared it with "teacher" to make sure that she knew where time was. Ahh idioms... useless little expressions that completely send our ESL friends on flips.

So my friend looked up the history of the word.
Basically, the idioms "keep track of time" or "lose track of time" relate to track as in a railroad track. Most rails were single tracks, so it took a lot of time-keeping in order to ensure that trains would be able to enter the station and exit without running into an oncoming train. If a train were late, it would run the risk of causing an accident - if a train ran into another, it would "lose" the track and either need to find a side track or be derailed. Being late on a railroad was never just a matter of fashionable arrivals.