You can save during a Scenario and Service, yes. You only have one "saveslot", so if you save again even on a different route the previous savegame gets overwritten. So always finish the Scenario you previously saved before saving a 2nd time. Please note that August 15th a new Update for TSW comes out and any savefile you make before this update wont work after the update. So make sure you finish the started scenario before August 15th or else your save wont work after the update. In some occasions resuming a saved game can unfortunately cause some issues, like disappearing markers and broken signaling. I hope that with the 2020 Update on August 15th they´ve redone the save function so that you can use it more reliable.
