1. Better to board Gaya HWH exp from Nawadah Stn. From Koderma jn...it is located about 65 km away. It is a straight drive on NH 20 for about 2 hrs. Buses are available.
2. If you want to go to Tilaiya jn...It about 70 km away in Hisua. From Koderma junction..go to Rajauli via NH 20, then to Hisua by State highway via Sirdala. I have never travelled by this routes. So I am unaware About vehicle availability.
So...I...
more... would suggest to board from Nawadah.