Hello,
1. Savegames are being stored in your steamfolder ….\Steam\userdata\…. search for a savegamename here.
2. For Trains i recomend a frequency of less then 10 min. The longer the game the lower the frequency should be.Roadvehicles frequency should be very low, i tried to keep it at around 100 sec .
3. It depends on the frequency and the passenger which will be left on the station.