Since the transit system is set for a bus route, is it better to treat a NPC horse/cart as a container object with a larger capacity in weight? Or treat a horse/cart as a "follower" that can be riden by a special command or verb?

Similarly I had planned on salvaging materials performed by switching pov to various characters...and all need to do things like siphon fuel (petrol), replace a belt on a vehicle, or just drive to one place after another.

They were going to move supplies to safehouse locations in order to resupply caches and use them in stages.

It depends on how it will play out. If the player uses the horse and cart to instantly get to a different location, then it is the same as a bus route. They may get complicated if you have a choice of vehicles, and also if you want to track the location of vehicles. I guess you would have locations that are car parks, and if there is a horse and cart there you can use it to travel to another car park.

If the player gets on the cart then moves in a specific direction to navigate across the map, a follower or container would be the way to go (essentially the cart is just a really big rucksack).

I thought you'd would just do whatever code you use for transportation, and then move the cart with you.

The syntax would be squirrely but I guess the vehicle types and animal mounts would be declared as people objects. A Goldwing carries twice what a human does. A horse could as well...unless it's a team of horses. You can bet a canoe would be handy...or a john boat. A humvee is bound to be extremely valuable.

I had hoped for speed or evasion but will settle for vastly increased bags of holding as well as unique ways to get from point A to B to C.

What? Are you saying basically create a rickshaw? Hahahhaha. No.