I spent 5 years taking the Toronto subway (Yonge line, North York Center to Union) every day. I noticed this interesting pattern. During rush hour, any small d

I spent 5 years taking the Toronto subway (Yonge line, North York Center to Union) every day. I noticed this interesting pattern. During rush hour, any small delay on one train will almost certain impact every train down the line- there's little time buffer between trains. The bigger the delay, the more trains effected by it. The more passengers per train, the more likely that train will have a delay- loading and unloading taking too long, sure, that can cause a small delay. But consider events like heart attacks, seizures, a fight breaking out- all kinds of major-delay-causing-events that are roughly speaking a linear function from 'time passenger is on the train' to 'likelihood of major delay event'. If you have twice as many passengers, you have twice the odds of a major delay. If a passenger spends twice a long on a train, you have twice the odds of them causing a major a delay. Delays cause more passengers per train and cause longer time-on-train for each passenger. It's all non-linear. Any one tiny delay can spark a total breakdown and the longer that delay is the more likely it will cause more in turn. How on earth does the system ever work at all?