Complex sentence: The driver (engineer) of the locomotive had been too preoccupied with his failed brakes to notice the absence of the cars until he reached the station at Saint Jean de Maurienne, where he finally succeeded in stopping his locomotive and its tender.
Split into simpler sentences: The driver (engineer) of the locomotive had been too preoccupied with his failed brakes to notice the absence of the cars until he reached the station at Saint Jean de Maurienne. Here he finally succeeded in stopping his locomotive and its tender.