PMEG diras la jenon:
Oni povas aldoni finaĵojn laŭ bezono. Se oni volas aldoni J-finaĵon aŭ N-finaĵon, oni devas unue almeti O-finaĵon:
- Tiu vorto estas skribata per du B-oj [= “bóoj”]. Aŭ: ...per du B [= “bo”].
- Sur la papero estis tri A-oj [= “áoj”]. Aŭ: ...tri A [= “a”].
- Mi partoprenis en tri IJK-oj [= “i-jo-kó-oj”]. Aŭ: ...en tri IJK [= “i jo ko”].
- Li elparolas la R-ojn [= “róojn”] tre forte.
USA is a big place with lots of regionalisms. Where I am from, at least, subway refers to the entire system, whereas train refers to the vehicle. I've never heard someone use subway to refer to the vehicle.
On the other hand, I live in a place where our train system is called the MAX Light Rail, but people refer to the individual trains as the MAX or as the specific train line (Blue line, Red line, etc). So, it wouldn't be out of the realm of possibility that people in different parts of the country refer to the train as the subway.
I haven't actually studied participles, but would "venanta" be more correct/precise? It's always bothered me that we learn "come" and "coming" as the same translation. I'd always assumed -- or at least hoped -- that there should be different words for, i.e. 'come' and 'coming', but Duo teaches us that 'venas' means both "come" and "is coming". (I know participles apply to more than just "venas"; it's just the example I'm using at the moment.)
Today, I found out why Duo teaches it this way. According to Evildea, it's acceptable to use, i.e. "vidas", to mean 'see' either/or 'am seeing'. Really took a load off my mind. I was kinda scared that we had been learning verbs incorrectly all this time. Thank you so much Evildea!!! https://www.youtube.com/watch?v=xn_jzwnYC-c
"La metroo ne estas venanta" is more precise for "The subway is not coming". (Note the presence of "estas" -- you can't say "La metroo ne venanta", which would be "The subway not coming". And "La metroo ne venantas" is possible but rather "experimental"/edge/avant-garde.)
But it's not "more correct" than "La metroo ne venas"; the participles are available for more precision but are not used as frequently as in English.