It is likely to be both ways:-
1> The bomb was planted in HWH trip CAR-SHED. (Since Falaknuma is a SCR train, SC does the RM).
2> Planting the bomb in a general coach in a highly crowded station like HWH. (Highly Unlikely but still can't be ruled out)
3> Bomb planted on-route from last night journey from SC-HWH. (Most Likely)
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what I think is that, option 3 is most likely, because trains are most vunerable at intermediate stations, especially stations with 2-3 minutes stoppages! How can you perform security checks if a train stops at a place for 2-3 minutes. It is also most likely not an important station, a less important one which is less secure. Then again, not all express trains and railway stations have RPF protection.
Planting a bomb in a terminal is highly unlikely!
And request #arya.rishav43 to add station tag HWH on the post.