1. You need to use double-decker trains, since they are the most spacious.
2. When a train reaches a station of destination and when all passengers leave the train, place the train to the station with waiting passengers. Do it several times until you have enough trains.
3. It is necessary that trains arrive to a destination station without interruption.
4. At some point you will need to research spacious trains. After that you need to start those trains. Sell normal double-decker trains and buy spacious ones.
5. Research acceleration of passenger boarding time (-10%,-20%, -30% of passenger boarding time).
6. You could also research powerful locomotives. Place powerful locomotives in every train and you will be able to add more spacious wagons into these trains.
7. In order to definitely win you can make trains with a couple of powerful locomotives and spacious wagons.
Most importantly you need to do everything right and play with maximum efficiency at the beginning of the game.