Does 了 have an evil twin?

Yesterday, sitting at the plattform of the Stuttgart train station, I thought to hear my son saying 火车走了 (well, the "火" got in a kiddy pronounciation accident).

I told my wife: "Technically he is wrong. The train is just going ("zou-ing"). It has not completely left the station yet."

But she insited that the sentence was right, because he did not say "了" but "啦":

 火车走啦!

I always thought 啦 is for mood only, but yesterday I learned it can also be a marker for continuous action. Only in conversional speech.  Taking the function of 正在. Although they are not fully replacible because you cannot say 火车火车走走.