During 2017 there are various new model trains available for the enthusiast, as well as the beginner. If you want to know about new model train releases it is worth browsing the maker's catalogue. Most of the catalogues are available online. If you are looking something specific to your set it may be hard to find. If there's enough demand for a specific engine it will not come out sooner or later though.
For the enthusiast, Atlas O's F7 Diesel are now available - both decorated and undecorated units. Menard's have a new container car for O gauge and various Railway Express Agency vehicles are now available for purchase.
For most people the go-to name is Hornby. They have two new sets available, i.e. the Hornby Junior Express Train Set, a new set for kids aged 4 and above, and the Mallard Pullman Train Set. New steam trains include the "Couer-de-Lion", "Leicester City" and the "Llanfair Grange". There are new steam train of every description from LNER, individual Welsh lines to late BR, so it's always a worth to check what new locos are available. There are also new types of coaches (Pullman, GWR Collett, etc.) and wagons (Ferry Van, Plank Wagon, etc.)
Like Hornby, Dapol have also created new engines such as "Normandy", "Caen" and the very similar 30089 and 30092 (black engines with BR emblems).
If you look on their website you can see a picture of an actual "Normandy" which operates on the Bluebell line in Sussex.
As far as Bachmann goes, there also have a Bachmann Branchline diesel. There is a number of Scenecraft buildings fitted with JustPlug, meaning that they can now be lit up at night. Many Bachmann engines are now programmed with DCC Sound making them produce the same sounds as the actual engines.
Fleischmann have a number of new engines, including the steam engine class 39.0-2 and the steam locomotive pr. T 10. KPEV.
Also this year there are Z scale locomotives and freight cars available, such as the General Electric ES44AC. There are also new types of signal decoder for such model train sets.
Oxford Rail has less items but they include a Carflat car carrier with the BR symbol on it as well as the Class 2301 Deans Good. Diesel engines include the cream-coloured class 225 DB AG and the strong red coloured class 215 DB AG. As with the above, they have new types of coaches and wagons and new types of controls.
Obviously the above is just a short selection, there's no way that all new types of products can be referred to in so limited a piece.