How do you deal with people cargo culting COVID-19 defense?

It seems like in your model, what happens is that the authorities switch from "Please wear masks" to "Please wear masks and avoid unnecessary talking", people nod along in unison, and a new social norm is created which functions similarly to the norms against talking too much in a library or movie theater.

I don't think it's that simple. For one, I don't think people would get it. I don't think many people are going to say "Oh yeah, from that one simple sentence I now understand exactly how much talking increases the risk of spreading covid, how important it is, what level we should tolerate and how we should punish it, how we should deal with people who are too lax/harsh on punishing others/etc". We're still struggling to coordinate on "How big a deal is covid?" norms on wearing masks and enforcing mask wearing. Given how much more inconvenient staying silent is,  I wouldn't expect norms against talking to be easier to coordinate on. 

If you remember back to the beginning of the pandemic, no one knew what to make of this thing, and so everyone was slow trying to wait to see what other people say before they decide what to think. This has all the obvious problems, but it's also worth noting that when people try to "think for themselves" you don't get a bunch of good answers, you get varied answers and dumb answers. A good example of this is Eddie Bravo, who is sometimes as being a brilliant jiu jitsu mind and has added a lot to the sport, but at the same time believes obviously crazy/dumb things. When you look at the debates he gets in about flat earth stuff, he actually makes better arguments than his round earth opponents because he's actually thought things through (albeit poorly) and his opponents are stuck trying to rationalize on the fly. Thinking things through from first principles only works when social consensus is less thought through than that, and while it worked for him in a new and niche sport, it's not that great for problems as tricky as "Is the earth round" let alone "How should we handle a pandemic". This sort of "follow the herd" mentality is necessary.

I don't know about you, but I find creating this sort of social consensus (and defying any existing consensus) to be stressful and I expect that most others do as well. In the beginning, for example, I remember really not liking having to be the one taking people from the comfortable mental space where they wanted to be and conveying to them that there's very likely a full blown pandemic coming that the entire world is unprepared for, and that they should probably start thinking ahead and planning accordingly. There were plenty of people I didn't even bother telling, because I didn't feel like I had enough social credit for it to be worth the effort. Once "We're in a pandemic, duh" had been established, it became trivial to convey to these same people "Keeping a door open and a fan on is probably much more important than washing surfaces", but that's because it's a smaller deviation from the accepted narrative and one that is emotionally "cheap" for them to consider.

When I put myself in the shoes of the authority having to say "Please avoid unnecessary talking", I anticipate getting a lot of push back. I anticipate people trying to frame me as a scaremonger, trying to ruin peoples social lives for my own political ends. I anticipate other people agreeing with me, either silently or in a way that further polarizes things rather than helps things. I anticipate it feeling qualitatively more similar to breaking through the ice to go swimming in a frozen lake than settling into a nice warm hot tub. 

Ice swims can still feel good with the right incentives and mindsets, and it would definitely be awesome if our leadership were more competent and motivated to find out what is the actual best course of action and communicate it credibly and understandably to a partially hostile population without raising their own defensive shields and mucking things up. At the same time, I think it's pretty understandable why even trivial things like "Please don't talk more than necessary" don't get asserted/communicated.

What you have described is a ban. It may not be a ban with criminal penalties, but "an additional sentence" in the official guidelines together with "social expectations that will reduce the amount people talk" is exactly what I was referring to when I talked about a ban on talking.

EDIT: Okay, I talked myself out of this. There is indeed a difference between a ban and a strong suggestion, and I'll allow that you intended to describe the latter.

The strength of my reaction was based on the fact that COVID-tide has already been extremely damaging to people's social lives, with many deleterious downstream effects, and inhibiting talking in trains is proposing to inflict even more damage just to shave off a handful of micro-covids. I don't think this tradeoff is remotely worth it.