I am pretty sure that próximo works just as well here instead of siguiente. See: Forum - Duolingo. But please correct me if I am wrong.
I came here to post the same thing.
I want to use próximo but am not completely sure. I’ll ask my profesor and report back.
Update: Apparently, siguiente is proper here because próximo would be used prior to the word tren.
I am still not sure about this (I took a bit of a hiatus…), as word placement shouldn’t matter. I think I can say siguiente tren or tren siguiente, for example. Do you have a source for your reasoning?
This is another post on próximo vs. siguiente: What Is the Difference Between Próximo vs. Siguiente? | Lingvist. Basically, it says that próximo implies the reference point is right then in the moment while siguiente is used when there is a previously-mentioned reference point. Maybe in this context of “next train” it depends on urgency?
Or maybe not. There are few comments here: Forum - Duolingo saying that próximo works just as well. Who knows! I guess I’ll find out when I go to Spain soon and have to take a “tren”…
I’m not a native Spanish speaker, but to me, “el tren próximo” sounds less than natural, a bit like “el tren cercano".
“el próximo tren” sounds natural though, and there are more examples of that than of “el tren siguiente” in other sentences.
When I was using Duolingo a lot, they always had it as “el proximo tren” (no accents on my keyboard). They would mix up “proximo” (or proxima) and que viene.quite a bit, with emphasis on where to place it. “Next week” (“La proxima semana”. And also, “la semana que viene”). "Siguiente and que viene coming after the noun, proximo(a) before.