How to handle transactions with SQLite and Android?

I try to include transactions in my application because writing in database is very slow and I saw here and here that transactions are a solution but they are still very confusing to me.

I have Schedule objects that contains an object LineStation, and I want to write them in database using transactions.

Here, the method addSchedules in my class ScheduleDAO, that writes all schedules in database. It contains only one transaction.

public void addSchedules(ArrayList<Schedule> schedulesList) {
    SQLiteDatabase db = this.dh.getWritableDatabase();
    db.beginTransactionNonExclusive();

    for (Schedule schedule : schedulesList) {
        ContentValues values = new ContentValues();

        // insert linestation
        LineStationDAO.getLineStationDAO().addLineStation(schedule.getLineStation());

        values.put(/*...*/);
        /* ... */

        db.insert(DatabaseHandler.TABLE_SCHEDULE, null, values);
    }

    db.setTransactionSuccessful();
    db.endTransaction();
    db.close();
}

And this is, the method addLineStation in my class LineStationDAO that saves the object LineStation given. It's called by addSchedules and doesn't contain transaction because it is "nested" in the addSchedules transaction.

public void addLineStation(LineStation lineStation)  {
    SQLiteDatabase db = this.dh.getWritableDatabase();
    ContentValues values = new ContentValues();

    values.put(/*...*/);
    /* ... */

    db.insert(DatabaseHandler.TABLE_LINE_STATION, null, values); // database is locked (code 5)
    db.close();
}

The LineStation insert implies an SQLiteDatabaseLockedException (database is locked -code 5). What I have done wrong, please? Thanks.