Also, is there a max to the max number of duplicates in a composition? And a max for the total number of vehicles in a train?

There is a 30 units limit per composition. Those 30 can be the same unit if you wish, you can repeat any unit as many times as you want.

About not including some features because they do nothing, keep in mind the mod spec is ahead of the game at this point, and some things like the country tags or the model description text are not displayed, but they will in the future. If you add them now they will automatically show up when supported.

引用自 Weird and Wry：

About not including some features because they do nothing, keep in mind the mod spec is ahead of the game at this point, and some things like the country tags or the model description text are not displayed, but they will in the future. If you add them now they will automatically show up when supported.

What I mean is which ones will the game actually get angry about if I leave it out? Because for example, on fictional trains, your not going to get anything to do with years or countries, so are those required?

In general all the metadata is optional and its absence should not crash the game or give errors. Just make sure to never have blank values in the mod.txt file. "countries_built=" is not valid, if there are no countries_built, then do not write a countries_built line at all.