"could... but" structure and more

Posted by Sphinx on September 11, 2003

Hi folks,
Could you kind give me some advice on these?

1.I'm puzzled by the "could... but" structure here!
"Though miles apart, could men but live forever dreaming they shared this moonlight endlessly!" wrote Song Dynasty of China poet Su Shi in his well-known poem.

2.I saw this kind of discount ad:
"Sale rail: everything less than $10 here!"
What does rail mean?

3.Is "Dirty Look Special" a kind of staring eyes?

4.Do you think "is" in "All this is can be done" should be removed?

"So far, the poplar, eucalyptus, apple and coffee trees are among those being engineered. All this is can be done today because we have a better understanding of tree genomes."