def filter_by_rectangle(lower_left, upper_right):
    with open('sbb.csv', 'r') as stops, open('filtered.csv', 'w') as filtered:
        filtered.write("Name,Latitude,Longitude\n")
        for line in stops:
            values = line.split(';')  # Semikolon!
            name = values[0]
            try:
                lat = float(values[1])
                lng = float(values[2])
                if lat > lower_left[0] and lat < upper_right[0]:
                    if lng > lower_left[1] and lng < upper_right[1]:
                        name = '"' + name + '"'  # Namen in "" einpacken
                        filtered.write(','.join([name,str(lat),str(lng)]) + '\n')
            except ValueError:
                pass

thurgau_ll = (47.37956193869952, 8.63275694257301)
thurgau_ur = (47.701252984008455, 9.451238355155777)
filter_by_rectangle(thurgau_ll, thurgau_ur)