Using SQLite in Python: database locked

I am trying to use an SQLite3 database with Python. My database functions are defined within a class and look like this:

def insertStation(self, stationID, nameNL, nameFR, nameDE, nameEN, latitude, longitude):
    c = self.conn.cursor()
    station = (stationID,
               nameNL,
               nameFR,
               nameDE,
               nameEN,
               latitude,
               longitude)
    c.execute('INSERT INTO station VALUES (?, ?, ?, ?, ?, ?, ?)', station)
    return

def insertTrain(self, trainNumber, commercialType, origin, destination):
    c = self.conn.cursor()
    train = (trainNumber,
             commercialType,
             origin,
             destination)
    c.execute('INSERT INTO train VALUES (?, ?, ?, ?)', train)
    return

def trainExists(self, trainNumber):
    c = self.conn.cursor()
    rows = c.execute('SELECT EXISTS (SELECT * FROM train WHERE train_id = ?)', (trainNumber,))
    if (rows.fetchone()[0] is 1):
        return True
    else:
        return False

def commit(self):
    self.conn.commit()

In another file, I am calling these functions in the following manner:

trainID = #some ID
if not(database.trainExists(trainID)):
    database.insertTrain(trainID,
                         schedule['CommercialTypes'][0],
                         schedule['Origins'][0],
                         schedule['Destinations'][0])
routeID = database.insertRoute(toUnix(schedule['DepartureDate']),
                               trainID)
database.commit()

When I run this code, it gives me an error saying the database is locked. How can I get this to work?