What does this mean?

What does ‘’ if they had known but why ‘’ stand for here ?

Is it like , if only they knew ? some kind of unfortunate omen .

Both were young fellows, both were rather poorly dressed, both had remarkable faces, and both were evidently anxious to start a conversation.
If they had known but why, at this particular moment, they were both remarkable persons, they would undoubtedly have wondered at the strange chance which had set them down opposite to one another in a third-class carriage of the Warsaw Railway

In meaning, it is the same as “If they had only known why…” Stylistically, the original phrasing is more poetic.

‘’ If they had only known why ‘’
And this means …

Crudely, it is the same as “If they had known why…” The inclusion of “only” adds emphasis by implying that something undesirable resulted from their not knowing.

They did not know why they were both remarkable persons, so they did not wonder at the strange chance …

The inclusion of “only” adds emphasis by implying that something undesirable resulted from their not knowing. ‘’

AH , as i expected, thank you man .

I see , thanks .