As far as I can tell, here are the "rules" for modern Lego Trains:
There will be two motorized play sets in production at any given time: one introductory and one advanced. For the past several iterations the introductory set has always been a passenger train, and the advanced set has always been a cargo train.
There is generally one motorizable train in production for AFOL train fans. The most recent of these have been Emerald Night, Maersk Train, Horizon Express.
Themed sets may contain a train independent of the above rules. Hogwarts Express, Constitution Train Chase, and Ghost Train are the most recent examples. These will start and end along with the rest of their theme.
Accessory sets like train stations and crossings seem to be related at introduction to the "core" introductory and advanced sets, but EOL is harder to predict. For example, 7936 went out EOL unexpectedly.
Parts sets like tracks and switches seem to be effectively immortal.
For Duplo Trains there are generally three in production at any given time:
A basic motorized set with engine, one car, simple track, and a few bricks. Examples: 10507, 5608, 3771.
An advanced motorized set with engine, two cars, advanced track, extra play features: Examples: 10508, 5609, 3772
A push train: Examples: 10558,5506,3770
Themed Duplo trains are less common than themed Lego trains, but they do happen. Examples: Toy Story, Thomas