Depends what you're trying to do. You can just place the flat piece of road from the library if you need a flat road segment. Then you can snap the node to your previous track segment by doing this:
https://psteamcommunity.yuanyoumao.com/workshop/discussions/18446744073709551615/1661194811510331228/?appid=233610

Also, if you just select any road segment that has an empty node, and place a new road segment from the library, it will automatically attach to the unused node.

My post title was not a good choice. All the assets i see curve, or undulate. I just want a nice flat straightaway. The omly way i can find to do this is to switch to global axis and align it with the rotation gizmo. Is there a better way?

There is a flat piece for all road types. Here's the one for the standard Empire road: https://i.imgur.com/3fLCCJf.png
It's near the bottom, so scroll if you zoomed the library panel in. Right next to it is one that tilts 45 degrees (thumbnail image looks the same), don't hit that one.

Thanks!