Sometimes, updates are available on the DLS.
In Content Manager, select the package that shows updates available.
Right-click and view asset versions.
This will open up a new Content Manager window...
Download any update available from the DLS.
Recently, after SP5 came out, there has been a bunch of asset updates. I downloaded a gazillion so far this way. The only assets that won't have any updates will be consists or trains. Other things such as track parts, buildings, industries, etc., will be available for download. In particular, there have been a lot of updated textures.