Opinion on Community Trains?

There's plenty of other factors, as well. Groups can sometimes do dedicated batches, the amount of active bundles (and thereby, spare keys) typically is a significant factor, and the nature of the train [eg, if it has a theme, like Christmas or Positivity or "let's explore locations in Europe"] can affect how compelling it is to potential participants.

I'm not in any way trying to say that other potential factors such as diminishing contribution rate aren't in play, I'm just noting that any claims towards such being the absolute cause of things [this is directed towards the claims of certain pessimists on the site, not your post] should instead properly acknowledge that other factors are likely involved, to some degree (regardless of whether that influence ends up being small or large).

In short, all of the following [and perhaps more] are likely contributors to how well a community train does:
Timing (as it relates to active deals, to seasonal compulsions, and to periods when people are more free for engaging with the site), Appeal (of theme), Coordination (of gifting groups), General Site Activity, and the Current Level of Community Investment in the Forums.

For my part, I didn't participate in the last train due to the timing (being that I was so busy with RL stuff that I barely managed to scrape together some time to go through the train at the very end of its run [and that being after many carts had already ended], nevermind having time to add anything myself) and the lack of conceptual appeal. I can't speak as to how that reflects the circumstances of anyone else, of course.

Trains run during the winter holidays should theoretically be the most succesful, given how people are generally less burdened by RL stresses [my condolences to those with rough family circumstances], usually have more free time, and are perhaps more inclined to make purchases (especially among those who feel compelled due to the nature of the holiday season).

I'm kinda flabbergasted by the last train taking place in September, but perhaps that timing works better for others than it does for myself, and the lack of theme isn't as prominent a factor as it could be. Guess we can't really tell without a same-year December~January train to compare it against. :P

[Insert opinion here] number 5 :P