Translation:The former train station still stands today.
From an American point of view, I also agree that most people would say "old." "Former" would probably used in more formal writing. "Ex" is really awkward and should probably be reported.
This ambiguity seems to exist in German too. I'm in Germany right now, staying in an old (former) string factory that has been converted into apartments. It is called "Die Alte Saitenfabrik"
I cannot tell you why but rather the state of the situation. There is a system how the adjectives are altered based on the gender and the case of the noun. So, for example when the trainstation isn't yet known it would be: Ein ehemaliger Bahnhof
Genitive: Eines ehemaligen Bahnhof
Accusative: Einen/den ehemaligen Bahnhof
Dative: Einem/dem ehemaligen Bahnhof
Or dative without the "einem/dem": ehemaligem Bahnhof But I'm not sure when that would be used.
See more info: de.wiktionary.org/wiki/Flexion:ehemalig
There are more examples of the declinations for different genres.
From the original German sentence (Der ehemalige Bahnhof steht noch heute .) can we know if the train staion is still in use? Or only that it exists but we don't know if it's in use as a train station? Sorry that this may be a strange question but not beeing native english sometimes it gets extra hard.