Things become more clear if the sentence is broken down:

山 + に (mountain + marker indicating in or on) = on the mountain + は (topic marker) = regarding "on the mountain"

then

どうぶつ (animals) + が (subject indicator) + あります (there exist [the subject]) = there are animals

Put them together and you get something translating roughly into "regarding on the mountain, there are animals" or more appropriately "There are animals on the mountain."

this makes sense now, as to what the は particle is meant for. I don't personally know of another language that uses the same grammar concept, at least not in the ones I speak

@leeviticus well that's the general idea anyway, there's probably a lot more nuance than that! I just wanted to give a general idea so people aren't as confused when it pops up in strange places

It's not that unusual though - in English we usually put the subject (as in, the focus) first in the sentence, or use vocal stress or constructions like 'as far as X goes' for emphasis when we put the subject elsewhere. The whole particle thing is unusual though, yeah - it's interesting!

@Medusa747 I think it's a subtle thing, the は adds emphasis to the 電車の駅に part, so instead of a simple statement of 'there are people at the train station' it's more like 'there are people at the train station'. There's an implication that there's something significant about it - maybe everywhere else is deserted, that kind of thing.

We use a change in speaking tone to add that kind of emphasis in English, Japanese can use particles instead. Way smarter and more knowledgeable people than me are talking about it here!

So would you say 電車の駅には人がいます if you want to say there's people at a train station?