Many many moons ago I was assisting a consultant for our company who was implementing a very complex rail system to move baskets of metal parts in and out of a 1500-degree blast furnace. The track itself was a fairly complex 'mini-railyard' on the shop floor that intersected itself in a couple of places. Several motorized pallets would shuttle baskets of parts around according to a schedule. It was very important that the furnace doors were open for as short a time as possible.
Since the plant was in full production, the consultant was unable to run his software in 'real time' to test his scheduling algorithms. Instead, he wrote a pretty, graphic-y simulator. As we watched virtual pallets move around on his on-screen track layout, I asked "how will you know if you have any scheduling conflicts?"
His quick answer, "Easy - the simulation will never stop."
