Pause the game when somebody gets off the train. Click on one of the people walking. It will show you details on that person. The details show which buildings the person uses in life along with which building is the current destination. Check to make sure that building is very near one of your drop off points. If it’s not, the person will not take the bus.
Also check your wait times. If the person has to wait minutes at a station for a bus, they will choose to just walk. The wait time plus the travel time must be less than the time it takes to walk.