I never understood why, either. I think the NdeM consolidation happened around 1930. Nevertheless Interoceanic (FCI) and Mexican Southern (M del S)engines bore these intials long after these roads ceased their corporate existence. It may be that the NdeM absorbed several narrow gauge roads that ran in the north and west of the Valley of Mexico first, and may have absorbed these other roads more slowly, paying the British owners on an installment basis. I know the Mexican Government owned other roads.It absorbed the Mexicano (FCI's standard gauge competitor) about 1959. The Pacifico (The old SP de M line from Nogales to Guadajara) is still independent from the NdeM system. Interesting history note: In 1938 Pesident Lazaro Cardenas fired the entire NdeM management, making it "worker managed". It took less than a year for this large railroad system to grind to a complete halt.