Something I found on SteamDB: SteamDB Unknown App 1804512 (Product FiveHundredFiftyFour (1804512)). Anyone knows what DLC this might be?? Look at the history: https://steamdb.info/app/1804512/history/ Anyone willing to take a guess?
Pretty sure I read somewhere (for the life of me and with no luck Googling it I don't know where) that it's a Just Trains product, there is every possibility I'm remembering this wrong and without being able to jog my memory via search results I wont swear to it though. Edit: I am wrong, disregard.. (see atomicdanny's post further down)
It's an unreleased dlc, but equally it has that name on purpose, it could be either 3rd party or first party but most likely the former. Before release all products have a similar name while under test or prior to release.
I have two similarly named DLC's in my library though, probably the top one is JT hence the reason I searched for it before and why I thought the OP's queried DLC was familiar/similar.
Product TwoHundredFiftyOne (500213) = Just Trains MK1 Coach Pack Product ThreeHundredSixtyEight (834409) = Class 159 South West Trains (came with the NSE 159 I believe) (they are both released so I can say what they are ) The one the OP mentioned hasn't been released or announced (it's a 3rd party content )
3rd party content? DTM's new BNSF project. Calling it. Kidding, it's probably too early for that. But I was talking to the guy there and he has a number of projects on the go, so I could actually be half right. My suspicion is this, because the images posted on December 20. I doubt I'll get a confirmation of that, though.