I personally think that the Southern Railways steam routes have been overlooked in previous DLC's for TS2018 which is why I am suggesting the Southern Railways WEST OF ENGLAND MAINLINE which would run from London Waterloo via Woking and Basingstoke to Yeovil Jct. (Including the Yeovil Town branch). I also think that this route should be set in the 50/60's to showcase a variety of loco's and rolling stock such as the Battle of Britan, Schools, Mougals, Merchant Navy, 2-BIL, 4COR and even Westerns just to name a few. I feel this route would have limitless possibilities for scenarios by recreating famous Southern express services eg; The ACE (Atlantic Coast Express), The Cunard and The Devon Belle, Alongside the commuter services to and from London Waterloo. Included in this route would be some notable landmarks including Nine Elms MPD, Clapham Jct, Battledown Flyover and Salisbury train station.
This would certainly get my vote. FYI, SteamSoundsSupreme have a freeware Southampton to Weymouth set in the steam era. I recommend it.