Second thumb rule of 500 kms would apply in case you intend to take a break journey.
If the difference between your arrival time of the train from A to C and subsequent departure timings from C towards B is more than 24 hours, then you would need to make sure than you have had traveled for 500 kms or more from A to C. Subsequently total distance traveled should also be more than 500 kms.
In case, the difference is less than 24 hours, the clause of 500 kms. would not apply.