Asinus

Oh, of course. Forgot about those. I only did some rowing on a canal for a year while studying. Sadly other than that, there is not much water nearby

Thats a good one. Do you have horizontal traffic lights, or what is the connection to the red light?

True. That's a flaw with the english words.

Yet the exit would always be on the stationboard side (left or right). So you would never know which side of the train to exit.

I'm not sure. It has always been done like this here.

I think it's just a convenience thing. Might be useful in crowded trains, so people know which doors they should not stand in front of.

Maybe its the wrong english word but portside is shorter than "left in the direction of travel".
Even more in my language.

If you want to include logic in the matter, then yes; you are absolutely right.
But i do like nautical terms.

Because it's a (barely) established abbreviation for exactly those directions.