I got 48 audio tracks for a mixing project.
 
I imported them and they all positioned themselves at bar 1801!
They also put flags into the time bar, so what I want to do is remove the
unwanted 1801 bars so that both audio tracks and flags move up to
the start of the song.
 
Unfortunately, if I select the unwanted bars, the Edit->Delete menu
is disabled. It is only enabled if a bar has something in it.
 
So how can I get rid of the leading empty bars (without losing the relative
positions of the marker flags)?

Select everything from bar 1801 through the end and slide.
 

Insert an event in any track between 1 and 1801, then Edit > Delete shouldn't be disabled.

jpetersen
I got 48 audio tracks for a mixing project.
 
I imported them and they all positioned themselves at bar 1801!
 

If all the clips start at the same time, try re-importing with "Always Import Broadcast Waves At Their Timestamp" disabled n Preferences > File > Audio Data. All the clips will import at Now instead of the timestamp in the clips.

Ah. That was it. Thanks!

...but I do wonder why such a simple thing should be impossible.
...Feature request. I'll put in a feature request.

Yes, I often find myself baffled as to why I can insert 8 bars without a problem (ok, usually without a problem - modern versions of Sonar like to only insert it on selected tracks which is never what I want) but I can't delete 8 bars quickly and easily.

jpetersen
So how can I get rid of the leading empty bars (without losing the relative
positions of the marker flags)?
 
...but I do wonder why such a simple thing should be impossible.
...Feature request. I'll put in a feature request.

It's not impossible at all, there are two main ways to maintain markers in relationship to other material. You can  also shift groups of markers or all markers independently. See this post.