"The subway is not coming."
Translation:La metroo ne venas.
So it wouldn't be confused with the Esperanto metro, a meter - the measuring unit.
As for metroo, it is a loanword metro, and -o is the noun ending.
Can "metroo" be used in Esperanto of an individual train on the underground, as seems to be the case here? It certainly doesn't work in British English. No one, waiting in vain for an underground train, would say, "The underground is not coming".