Apologies for the double. I did a full overhaul of my cache and asset database, and found that there is indeed a missing asset from the route in build 57898, making the route unusable for multiplayer until the missing asset is uploaded to the DLS.
Unknown Location: <kuid2:68787:35071:2>
Installed version: Gaense,<kuid:68787:35071> (by the way, faulty in build 57898)
EDIT: It's on the Black Pages, you have to check all the boxes to see it.
EDIT2: Installed the asset, cleared all missing dependency warnings, still no dice. May I remind you, this is the copy of Trainz that has no local modifications to any content (faulty assets or assets with faulty dependencies are deleted; no third-party content).