Infinite train prefab with randomized cars, and another that alternates sending a health car and an ammo car, with lights to indicate when the next one is coming.
Both prefabs also include logic to make sure that it doesn't send too many blocking cars in a row, or too many passable cars in a row.
Something to consider for designing with this, The rockets car is designed to allow players to climb from ground level to the top of traincars, so design accordingly or remove them (If you do, make sure you remove it from the logic_case entities too or it will break!)
Game_text is there to explain things; Read their comments, and delete them if you copy this into another map, as they don't actually do anything in-game.
Trigger the logic_relay named "train_start" to start the train's movement.
Unfortunately the health & ammo prefab breaks if you stretch it too long since it uses the same two cars every time. There's a couple ways you could improve this but I haven't. Maybe at a later date.
The basic infinite train, however, you can extend and add as many bends or whatever as you want and it should work.
Give me a poke if you end up using this, I'd love to hear about it.
Or if something's broken, please tell me.