2015 is not the year that Train Simulator makes its promised switch to the Unreal Engine 4. Instead, this year's revision Train Simulator 2016 [official site] brings a small set of updates and a new selection of content for the base game. Even as Train Simulator updates go, this year's seems pretty slim. An optimist might say that could indicate developers Dovetail Games are focusing on the new Unreal Engine TS, while a pessimist might say Dovetail are getting stingier. Me, I'd probably just invent two hypothetical people and use them to distract from my handwaving.
Train Simulator 2016 comes as a free update to TS2015, bringing a revamped user interface, better searching, and an update to the tutorialising Train Simulator Academy.
Train Simulator also comes as a new package you can buy, with a different set of routes and content included in the base game. You'll get a historical journey down the Cornish Riviera in the age of Steam, some freight-hauling through a harsh and snowy land, and more. If you already own TS, I believe you can still simply buy 2016 to get this new stuff added to your collection.
At this point, I'd turn to Steam user reviews to get an idea of how people are receiving TS2016, but Train Simulator is one of those fascinating games where people who've played it for hundreds of hours will tell you it's awful. Everything I've heard - and the little I've played - says TS has a whole load of problems, but it's the biggest game in a niche market where people don't have much choice. One can play it while disliking it because it's the best you'll get. Also, it must be hard to not feel you're being mugged off when the base game is shoddy yet has literally thousands of pounds of DLC. Hmm!
In closing: if you want to get into driving virtual trains, this is wonky but one of your better bets; if you've played TS before, you know the score. TS2016 is on Steam for Windows at £25.49.