The Boring Company tunnel is supposed to carry 4400 passengers/hour.
Assume 2200 in each direction and an average of 3 passengers per car, that’s 733 cars per hour going through each tunnel. That is one car every five seconds.

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At 30 mph it takes about 50 secs to travel between stations, so at full capacity, there will be 10 cars in each leg of the tunnel.
Suppose one car is disabled, say car number 3. Cars one and two will carry on to the station, cars 4 to 10 will bunch up behind the disabled car.
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More cars will enter the tunnel, depending on how long it takes to report the disabled car and stop the traffic.
It is possible that cars will continue to enter the tunnel until the last car becomes visible from the tunnel entrance.
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Now suppose that the disabled car is on fire. There would be a long row of cars, with passengers and drivers, trapped in the tunnel. There is no emergency ventilation system to blow the smoke away from the cars.
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There is no pedestrian walkway, the only way to safely exit is to reverse in the car through the narrow, winding, smoke-filled tunnel. It is easy to imagine the panic and maybe a second accident blocking the tunnel from the entrance.
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The scenario I describe may be unlikely, but it is certainly not impossible
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