I hear this from time to time but it doesn’t pass the sniff test. If we assume the vehicles are points, their flow is precisely determined by stopping distance (as measured in seconds). If on the other hand we assume the vehicles are very long, their flow is determined by speed. Thus flow at minimum does not change with respect to speed, but in practice increases with respect to speed, assuming time-constant stopping distances (i.e. you pass X point Y seconds after the vehicle in front of you does)

That means that the slower you go, the more parcels per seconds you will get, because the breaking distance increases more quickly than the velocity!

Of course this is a simplification, Because this assumes the trucks are points. The best case of this model would be an infinite density of non-moving trucks! In reality trucks have some length, which means that they can not be infinitely close, so 30kmph is probably close to the optimum.

Which is why real world freight trains have stopping distances at 65MPh of over a mile. This is also one of the reasons they can’t climb seemingly mild inclines.

Limiting the underground freight trains to say 1/10th a mile and maximum throughput is probably close to 30mph.

Ah yes, the "sniff test", in which things which are untrue are assumed, which make the interlocutor sufficiently confident to pontificate. "Sniff test", as a concept, doesn't pass itself, rhetorically (-- or maybe it's a "code smell").

That's an extreme case of rail line, but "much higher than any rail line" is kind of extreme phrasing too :)