As already discussed on the forum and also explained in the manual, the number of trains depends on the estimated number of passengers, which is calculated dividing the total stations load by 220 lbs, which is a rough industry standard of how much a passenger + baggage weights.
This means, to get the correct number of trains, you need to load your airplane properly. If you start a flight with the default 737-800, it starts 100% full by default.