Build train station after train station connected together. When you build the rails that connect them underground, that looks like a tunnel. Build several attractions, stores, and landmarks, on the open ground level upwards and downwards to the rail tracks and all train stations connected to each other, so that in that area, the passangers will exit trains underground, and so they can spend a while on those attractions, stores, and landmarks. Since they can only access those through the underground, when they exit the passanger trains, the station automatically builds malls underground. Those Underground Malls, are counted like this: Firstly there are build two at once. They are black colored, and are aligned left and right. The bigger the demand is, the more and the faster another two more Underground Malls are build adjacent to the other twos. Schedule the passanger train so that the passangers can exit the train on the station you wish for Underground Malls to be built. Once you have about ten or more Underground Malls on one train station, you shall schedule the passanger train to stop and let the passangers exit on the next station in the underground level. Proceed like that, until you have enough Underground Malls built. The other four trains carry the necessary supplies in order to make it possible to build those required attractions, stores, and landmarks in the level on open ground. Have the loading platforms for that, in either the first or second underground level, also in the exact area, but above or beyond where it is required, so that they can work on buliding by using materials. I am sure you will understand now how this works. It is all about logic. Also, figure out how to schedule the trains - including the supply carrying trains, they as well need to be scheduled. This is much more easier, if you do this step by step, after successful progress, when a desired amount of Underground Malls have been built on a station for you. Now, if you still have any complications, feel free to explain yourself, and I try to help you out.