When a freight train is connected to a line of cars, there is a little "give" in each connection. Just that inch or so that means that when the engine starts, it is only pulling itself for a brief moment, and then only pulling itself and one freight car, and then only itself and two, etc.
The same is true of braking. When the engine hits the brakes, it is only braking itself for a second, and then only itself and the car directly behind it, and so on. There is that tiny interval of time before the crushing weight of the entire train is hitting the engine.
What probably does not happen in real life is that the engineer applies the brakes, expecting that crushing moment, and instead just comes to a stop instantaneously. The freight got lost somewhere. Nothing is coming.
I am losing the metaphor but I guess the conductor might wake up a few hours later shaking in bed. Maybe you do not always know what you are hauling but it's disorienting to find it was nothing.
(I am consistently bad at imagining how much rejection will crush me. I am not sure why I imagined I would quickly be fine with getting rejected by my dearest friend. I wish I had said even one of the ridiculous things I had been turning over in my head for months, but I guess "I am deeply in love with you" got the job done.)