It makes trains placed by bots switch to automatic mode as soon as they have been completely built. And that's all the mod does. Blueprinted trains contain the schedule of the train they were copied from and fuel (vanilla features) so it will join its siblings in whatever heavy hauling work there is as soon as it is born :)
This mod and this BP makes it possible to make a unique ID generator:
It's a tiny mod that only does a small check in the event on_train_created so its performance impact is absolutely minimal and will not impact your UPS at all. Mod name suggested by eradicator
Works on straight rails (north/south and west/east) only because that's simpler. Actually, only the first 2 and last 2 train sections need to be on straight rails, the middle section can be on diagonal facing or curved rail.