1. The simplest, easiest, and current role: just make them a small, cheap, low capacity player carrier with a few item slots.

2. Like Traincraft, allow players in the caboose to access the GUI of a standard vanilla crafting table and furnace. I personally dislike this as it lacks realism in terms of furnace functionality and lacks customizability in terms of the energy sources and mechanics of smelting.

3. An augmentation style system: place any single block machine or crafting interface inside one or more of the caboose's machine slots and be able to access its GUI. Fluids and energy would probably have to be inserted into the machines from player-held containers if they support it.

4. Something unique, even if unrealistic: For example, HP regeneration or status effect nullifying effects under certain conditions, ability to stop and brake all connected cars and locomotives, an interface specifically for crafting of small track parts, or ability to view the amount of items, fluids, and players in each connected car and locomotive.

Bumping this for attention and extra input

I strongly support WW's points 1 and 2 in keeping them very utilitarian with mixed cargo and passenger capacities, I'm also all for the idea of making them into portable furnaces and crafting tables to further enhance their usefulness.

I also think that stock of this "class" should be able to have a slightly modified locomotive's gui, removing the throttle while still keeping the speed readout and brake slider there to make their function more in line with how cabooses and brakevans were prototypically used.