Regarding vehicle upgrading, what you're looking for is autoreplace
, rather than autorenew. You can of course try to optimize your routes to avoid having a huge number of vehicles but autoreplace facilitates the process either way.
Regarding food and distributing loads among stations, you might be able to do that with conditional orders
. Perhaps setting an order to jump to a different order (i.e. go to a different station) when load percentage is X would do the trick.
Trying to find the best profit really only matters in the early and mid game when money is an issue, but at some point the game stops being about money and starts being about optimization and playing around with strategies, like making complex networks using signals and that sort of thing. So I wouldn't worry too much about hunting for the best way to make a profit, try to do what seems fun to you or fits your style of play. It's essentially a sandbox, after all, and I mean, at the end of the day, if you really wanted quick legit cash, all you'd have to do is use a well-placed aircraft route and game the system, so...
Regarding city size, I find that turning off cities building roads and doing it myself, or strictly using the grid city road layout helps control city expansion a bit. It's also important to keep in mind that if you want to run a metropolitan train system, you have to plan ahead and place the stations before
the city grows larger, as there's no underground in OpenTTD (the closest you have are tunnels but you'd still have to dig land inside the city for their entrances and you need space for the stations). That or you'll have to rely on trams and/or road vehicles. Alternatively, you can always go "creative" in an existing game with an existing city if you don't feel like restarting and just magic dynamite some space inside the city for stations, but that requires using the cheat menu. It's an option nonetheless.
Path signals really help make rail networks more efficient by dramatically reducing the amount of infrastructure and vehicles you need to use. You don't even need to go complex with it, either. A simple one-way circuit is already a huge improvement over several point-to-point independent rails for passengers and mail, for example. I find basic usage of path signals is fundamental to playing with trains properly in this game and actually enjoying it.
If you want the game to distribute cargo and give it destinations you could always enable cargodist
in the settings.
AI are no longer included by default with the game, including old TTD AI. They've been redone entirely and you can now find them online to download and set them up in the menu. Check the in-game content download service
and find some AIs (and their required libraries) in the list and try them out. You can also find community-made scenarios and heightmaps (to load in the editor as a blank generated map) in the service, by the way.