If you want nice visuals and more buttons to press but you're less fussed about the trains and routes you drive, go for
Train Sim World 6 (TSW6).
If you want realism, variety and the ability to easily mod stuff and create your own scenarios, go for
Train Simulator Classic (TSC).
Either way, you'll generally need 16GB+ of RAM and a half-decent GPU to get a good experience on either (if NASA have something spare then that's always my go-to choice) and you might want to consider taking out a (second) mortgage to cover the cost of the game and all the route+train DLCs.
Some TSC advice:
Although the creators of TSC, Dovetail Games (DTG) made
a GEML route 12 years ago, it's pretty dated and has been superseded by newer stuff (see below). In the east of England, most of the routes+trains are made by third-parties (don't know why); Armstrong Powerhouse (AP), for example, has the
Wherry Lines, the
Class 156,
170,
315,
317 and
321 various packs for the loco-hauled Wherry+GEML services, and some good
enhancement packs for the game. If you've got a couple of kidneys spare, Alan Thomson Sim (ATS) has
Cambridge to Peterborough,
King's Cross to King's Lynn and
the GEML, plus a
Class 745 and
Class 755. Bear in mind that many third-party add-ons (especially ATS stuff) has a lot of payware requirements that you need beforehand.
Hopefully that covers most of your questions; feel free to ask if you still have any.
