This is a relatively quick train loader for 1-4 trains. It uses red transport belts. The loading times are less than 35 seconds on a short track. The time may go up, if your train makes really short trips (Less than 300 units towards the goal; Also: makes no sense to use a train if the trips are short).
To get the short loading time, you need 4 fully compressed ore transport belts. Also let it circulate a couple of times so that everything is in order.
If you have short train routes, you can limit the chests more than I have done. If your train is gone for a longer time though, then you can take out a couple of the limiter cells from the row of chests, that are closest to the train tracks. I suggest limiting all but 3, since one cargo wagon can take 2k Iron ore. We have 12 Inserters (6 on each side), that means that each inserter needs to put in (2000/12 = 166.666) 166.66 Iron ore. If we limit it to 3, the chests can save 150, and the conveyor belts will also be full. This means that the chests wont overload, but will have almost the exact amount you need. IF THE BUFFER CHESTS ARE FULL, THE TRAIN LOADS IN ~6 seconds
Thanks for downloading a have fun!