This is one of those comments that is annoying to respond to because it may be technically correct because the original comment wasn't worded ever so precisely, but we all know that "does not increase safety" is not an absolute statement. You could strap a pillow to your chest and get ready to jump out of an airplane, and I'll say "that won't cushion your fall," but "technically, it will," you argue.
Besides--reduced capacity causes platform crowding which is dangerous and displaces passengers into other modes of transportation like foot or automotive traffic, both of which are more deadly per passenger-mile than the subway.
Enforcing the speed limit the system is designed for intuitively sounds like very reasonable and effective safety measure, not comparable in any way to your fantasy scenario.
The whole point of having an analogy in the first place is to sacrifice accuracy in order to illustrate a point in a more obvious way. Analogies are often fantasy scenarios. And making arguments about how things “intuitively sound” is a big step into a world of unscientific thinking.
The question here is, “What is the relationship between train speed and safety?” My guess is that the risk has a bathtub curve to it. At low speeds, certain factors dominate, the dangerous effects of reduced capacity. At high speed, other factors dominate, like crashes. This type of curve is found in many different types of systems. This curve exists in automotive traffic, where both high-speed and low-speed drivers are known to be dangerous, and speed limits that are either too high or too low make the road more dangerous. With an actual study, you could try to measure the curve. Perhaps you would find a large, flat area where changes in speed have small or even negligible effects on safety.
Kind of like the negligible effect on safety that wearing a pillow on an airplane would have.
But we don’t know the answer to the actual interesting question, do we? We don’t know what the exact effect of speed on train safety is. We know that train safety is very different from car safety—I was just looking at some statistics comparing them. But here we are, dodging the real question and complaining that an analogy isn’t realistic enough.
Besides--reduced capacity causes platform crowding which is dangerous and displaces passengers into other modes of transportation like foot or automotive traffic, both of which are more deadly per passenger-mile than the subway.