@Alphabets supply ships and care packages are expressly for tactical benefit, through camping or securing the loot inside. They make excellent hot-zones for fast-paced heavy fire combat, and spawn during times when players will be rotating in the first place, giving players the choice of ignoring or moving towards "better loot".
The train on the other hand, is pure Darwinism, survival of the luckiest, whoever makes it off the train usually has the best loot. It's randomly tactically advantageous due to orientation, travel direction, and ring placement, and for the most part completely unnecessary to either loot or control. There's no point to it respawning with more loot, because the level 1 ring is going to lock off most of its stops rendering it inaccessible anyway.