Donīt just call them 3-digit trains: it is not only 3-digit trains which take extra time.

From Shanghai Hongqiao to Beijing South, there are 8 express trains daily. These depart at sharp hours 8:00 to 11:00 and 14:00 to 17:00. The other sharp hours do not have expresses - 7:00 (first train in the morning) and 13:00 are non-express trains, and no trains leave at 12:00 or 18:00.

The 8 expresses are numbered G2 and G4 (the 9:00 and 14:00 ones, taking 4:48) and G12 to G22 (the others, in order of time, taking 4:55).

The other trains take between 5:20 and 5:36. This includes all 3 number trains (G102 to G164) BUT ALSO the 2 number trains between G32 and G44. It is only the trains G22 and under that are expresses.

In your case, G18, departing at 15:00.