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.
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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.
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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.
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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.
FWIW, Google Ngrams says the following;
el próximo tren 0.0000019938%
el siguiente tren 0.0000010998%
el tren siguiente 0.0000002200%
el tren próximo 0.0000000498%
(I’ve been trying to post the link, but it just keeps getting mangled.)
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Results with ‘el tren próximo’ are likely just the people asking about it 
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