As noted, the general setup is the cell radios are located in the cafe/lounge cars (this is one reason why the Empire Service trains continue to drag around a cafe car, even when terminating in Albany. That and they stick business class there).
Last I heard, there were a minimum of 5 antennas per cafe car, and they were designed to hop from carrier to carrier as needed.
Each coach (and now I believe sleeper) has a wifi antenna that transmit to the cafe (I've been told somehow via wire, but I'm not sure how they do that unless they're piggybacking it on the HEP, which is technically feasible).
So, in theory, it doesn't depend where on the train you are (i.e. being further from the cafe car doesn't slow things down).
In practice, it can. As someone noted, the Amfleet cans aren't quite conducive for RF signals. This isn't entirely true, but it's not false either. The thing is, for various reasons, the WAP (wireless access point) within a particular car might be off-line, which means you're trying to pick up a signal from the car in front of behind you. With the amount of metal at the end of the cars, that's going to be an issue.
The real issue I've found with Amtrak wifi honestly is that it HAS been so successful. I've seen several memos over the years talking about the various upgrades and it strikes me that they're trying to keep up with demand and it's been hard.
Yes, as someone noted, in theory, streaming is banned, because that's the most bandwidth intensive thing. But, there's ways around that.
But even w/o pure streaming, you can find that a 50+ people per car (to pick a random number) can quickly eat up bandwidth. On a 7 car train, that's 350 people trying to get access through the cell radios.
And it's unfortunately a self-reinforcing issue. On popular routes, like the NEC, you've got a lot of business travellers and other folks who expect wifi, so will use it. The minute you upgrade that, people will use it more, etc.
And as noted, there's stretches of the Crescent for example where you will get no wifi because there's simply no cell service. Even using my own phone's data shows no service.
In my experience, planes partly do better because they lock down access FAR more Amtrak and because they charge for it, I suspect far fewer people use it.
Myself, I often try to use Amtrak wifi, but if I can't, will switch to my cell. It's not necessarily faster (which makes sense, if the trains in the area are swamping the cell network already) but a tad more robust (fewer dropped connections, etc). But even then, I often find both will work poorly.
In fact, I'd say that my data usage on the Empire service ends up being no more reliable nor faster than when I started using it about 15+ years ago, and I suspect that's partly because there's simply that many more people now using it that the additional bandwidth and cells "slots" have been filled up.
I'm ignoring details like NAT that may be an issue on trains (but shouldn't) and filling up MAC address tables, which I suspect is sometimes an issue.
That said, from what I can tell, their equipment is not consumer grade, but above that.
That said, I can offer some advice (which if everyone did would help).
Keep as few tabs in browsers open as possible. In fact, use as few apps on your phone or laptop as possible. You'd be amazed at the amount of data portable devices send back and forth on even "inactive" tabs and programs. This will reduce your bandwidth requirements and if everyone did it, it might help overall.
Finally, I suspect right now, if you were to ride on the NEC, you'd find the bandwidth to be very fast :-/
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