I've gotten this issue a few times with certain content as well - it turns out it's the content that I'm trying to download at fault. Despite appearing in the CMP list as something you can download for your version of Trainz, in actuality the content may actually be for a higher Trainz 'build' (ala, version) than the version of Trainz you have. Hence the software refuses to download it. As the content you're having trouble with appears to be a dependency for a route you've downloaded, it's very likely this is the cause of the problem.
I've been told the only way apparently to overcome it is to purchase a later version of Trainz, register it, and then try downloading the item again, but I presume you could also hack the route's config file (a text file that all Trainz items have) and delete the entries in it for that particular piece of content, but that's more of an advanced way of doing things.