"The subway is not coming."

Translation: La metroo ne venas.

[rwhubert]

why is it metroo and not just metro?

[mihxal]

Because "metro" means meter.

[llusx]

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.

[mizinamo]

Similarly with taksio from taxi.

[vikungen]

heroo from hero

[RomajiAmulo]

Is it always that way?
Or is it just often that way?

[mizinamo]

Not always; for example, "judo" is ĵudo and not ĵudoo.

[Cocio_16]

« La metroo ne venas »
Jen internacia frazo.

[deactivated user]

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".