I'm a non-native English teacher in Japan.
My question is whether the word "manners" is singular or plural.

Some says that
"Talking on the train is a bad manner." is right.
"Talking on the train is bad manners." is not correct.

Thank you for taking time...
In a dictionary I see...

"After you" is good manners.
It's not bad manners.
and so on.

So I think the word "manners" is treated as a singular word just like an abstract noun.