<Tr> <Td> oo </Td> <Td> Acknowledgment of any signal not otherwise provided for . </Td> </Tr> <Tr> <Td> ooo </Td> <Td> When train is stopped: means backing up, or acknowledgment of a hand signal to back up; when moving: stop at next station . </Td> </Tr> <Tr> <Td> oooo </Td> <Td> Request for a signal to be given or repeated if not understood . </Td> </Tr> <Tr> <Td>--oo </Td> <Td> Warning that a second section of a timetabled train is following . </Td> </Tr>

What are the notes of a train whistle