It takes 8 hours for the express train and 12 hours for the local train to go from a to B. one hour later, the express train and the local train run for a total of 2 hours

1 / 8 + 1 / 12 = 5 / 242 × 1 / 8 + 2 × 1 / 12 = 5 / 12. After one hour, the express and local trains took 5 / 24 and 5 / 12 respectively