The timetable system had several disadvantages. The first was that there was no positive confirmation that the track ahead was clear; only that it should be clear. This system did not allow for breakdowns and other such problems. The timetable was set up in such a way that there should be sufficient time between trains for the crew of a broken-down or delayed train to walk back up the line far enough to set up warning flags, fusees and explosive devices known as torpedoes, which alerted a train crew to an occupied track ahead.