Didn't knew Glorious Trainwrecks. They are imho indeed a vastly different thing.

Underground Game is a open term. If somebody would ask me if they can use it, I'll threw a short glance upon them - and either answer "Yes" or "Fuck you" (usually, not that direct - but clearly telling them what I would expect you to change if they want to be in any relation with me); the latter requires them to do something that I personally despise or reject (group specific forms of misanthropy, strong general misanthropy, exploitation of others). Such a dev might still call it a underground game, but where I can I will hinder it, I will - and I'll never recommend, rate, or recognize such a game. Games might pop up at mastodon or itch, or somewhere else using the term "underground game" (indeed, they did before I started). I might disagree with them being "underground games", but as long as I don't despise them, I won't bother to criticize their usage of the term by them.

Trainwreck is a existing platform, probably with a rather lax curation and no implemented means of monetization; it seems to be directed to people who enjoy rather raw, DIY-style games.

If one of the Trainwreck-Games would come up call itself underground game, and they could do so in their full right - because most of the games there seem to be indeed underground games. But for some I might surely say that they aren't "good" in the way I search for. Those might be underground games, but I usually don't recommend such games to others (if they don't have some spontaneous momentum that they captured, that is so good that it makes the whole thing cool - but this requires some mastery or small chances that are really hard to find by their nature if you don't have any hints).

I skimmed shortly through the games listed there, and most of them are honestly stuff that I would usually filter out when I search for games to play. Because games that are presented in this style aren't usually made to be played, but are mostly nice for the people who made them, and maybe even their friends and relatives. After all, these people are often proclaiming that they hammered them together in a few hours themself - and if you ever tempered with game making, you'll know what short amount of work two or even ten hours are; I guess it is not impossible to make a good game in such a time-frame, but I believe I have yet to see it (at least when it comes to video games: I played quite good easy non-video games that were made spontaneously).

Imho, good Underground Games should be something that the developers thought was worth to invest work in; they didn't do it to make profit, but because it was exactly what they wanted to make, regardless that it doesn't necessarily sells, and who stay true to this idea, often releasing it in a non-commercial space without much marketing blimbim (there are a few examples of commercial successful underground games; "The Sea will claim everything" is a wonderful example where I'm quite sure about this; iirc the dev was even afraid of losing their job over it. But these don't need much promotion from my side.). They don't always succeed, but they want to the good shit, and want make it right.

The Manifest, this "Sublemmy", among some other things, are attempts to initiate discussions about game development, and to "coin" underground gaming as a name that allows people who develop, play, and search for such games to find each other. Some are already here; if you recommend a game, it will be found by few, but cool people - that are somewhat likely to play it. Not sure if this can be done by Trainwrecks, but again - would have to take a closer look.

Hope none of this sounds harsh against Trainwrecks or their developers. I sympathize with them and their call, but I probably don't want to play their games.

As for jams, I see them as a "brain storm"-kind thing that tries to bruteforce good ideas. I don't like "brainstorming": Collecting many ideas and practically doing no critical reflection about those ideas ignores, in the end, moral and logical problems in favor of efficiency and a good look, and this is basically one of the main problems that the world suffers from - if you ask me. Nevertheless, I know some good games that resulted from jams. So if you disagree, do jams - and if they make you happy, ignore me; they are not the problem itself.