We know that Sylvia catches the train. There are two possibilities how this could have happened:

Bus on time:
If she had caught the train, and the bus was on time, probability that this would have happened
= 0.6 x 0.8 = 0.48

Bus late:
If she had caught the train, and the bus was late, probability that this would have happened
= 0.4 x 0.6 = 0.24

Total probability of catching the train = 0.48 + 0.24 = 0.72

Probability of catching the train with bus on time = 0.48/0.72 = 2/3