I also think they need a little more fine-tuning, but on the other hand I don't think they were made for short distances. But since all trains can use the same rail line, at least at the moment, the true power lies in its scaling potential:
A coal train with 4 freight wagons can carry 4 x 32 x 100 = 12800 Coal. One trip to my coal mine and back takes about 6 minutes, so the maximum throughput of the train is 12800 / 6 = 2133,... coal per minute.
You could now argue that 3 MK5 Belts do the same, but if you suddenly need double or ten times the coal (who knows what future updates might bring), then you can either build 10 times more belts across the whole map, which is annoying, or you just set up 5 more trains, which is done in like 5 minutes.