<Tr> <Td> </Td> <Td> Line 6 </Td> <Td> El Rosario (W) </Td> <Td> Martín Carrera (E) </Td> <Td> 11 </Td> <Td> 11.43 kilometres (7.10 mi) </Td> <Td> December 21, 1983 </Td> </Tr> <Tr> <Td> </Td> <Td> Line 7 </Td> <Td> El Rosario (N) </Td> <Td> Barranca del Muerto (S) </Td> <Td> 14 </Td> <Td> 17.01 kilometres (10.57 mi) </Td> <Td> December 20, 1984 </Td> </Tr> <Tr> <Td> </Td> <Td> Line 8 </Td> <Td> Garibaldi / Lagunilla (N) </Td> <Td> Constitución de 1917 (S) </Td> <Td> 19 </Td> <Td> 17.68 kilometres (10.99 mi) </Td> <Td> July 20, 1994 </Td> </Tr> <Tr> <Td> </Td> <Td> Line 9 </Td> <Td> Tacubaya (W) </Td> <Td> Pantitlán (E) </Td> <Td> 12 </Td> <Td> 13.03 kilometres (8.10 mi) </Td> <Td> August 26, 1987 </Td> </Tr>

How many north american railway metro systems are busier than mexico city's metro