It’s becoming a common theme for me to write about little frustrations. Hopefully this little frustration will save us both time, as I’m sure I’ll hit this again myself.
According the NetSuite’s Record’s Browser, this should work. And in fact it does… but only if you know ask nicely!
When you filter on “Mainline”, you must do it in a filter expression. A standard search.createFilter() blows up.

Every filter in the filter expression I show above works using search.createFilter(), except “Mainline.”