Generally bridge is constructed to connect two ends of valley or river or sea to access the train, buses and other vehicles.
And another term is called flyover (it is also type of bridge) is constructed on highway to regulate the vehicles traffic on particular rout without disturbing the underneath traffic crossing and some time across & over the railway line.
Now a days RCC as well as Steel both type of girders are used. But steel girders are preferred due following reasons :
a) It withstands more for vibrations than RCC. ( when train moves it create lot of vibrations)
b) The design of this particular span (which crossing over Indian railway territory) in accordance with Indian railways and they might have belief and safe experience with steel bridges or preferring as per their guidelines.
c) To do the faster & safe work without disturbing and diverting underneath railway movement.
if you want to know in more elaborate then pls. take the feedback from expertise specially in Indian Railway bridge domain.
Trains are pretty long and heavy and they generate high frequency vibrations when moving through the bridge.
Now a bridge can be made of steel or concrete. But the only drawback of concrete is its low ductility and resistance against vibrations.
Have you seen a jack hammer?
How does it break concrete on which it is used? Does it generate very large force? No. It vibrates very quickly and splits up concrete into smaller chunks. This is because of brittle nature of concrete and low fatigue value.
But steel is not brittle, it is ductile. It can sustain such oscillations and has higher fatigue strength. I am not saying that when train passes through the bridge, it generates very high forces, no..!! Concept of fatigue is completely different then the concept of non-linearity.
Take a steel wire and repetitively bend it back and forth you will notice that the application of force applied by your hand is reduced drastically after certain cycles. This is called fatigue. It is”Stamina" of material as how long can it survive in this endurance. Steel can take many more cyclic loading as compared to concrete.
When a train passes over a bridge deck, then the deck vibrates up and down. This vibration demands endurance. Thus steel is preferred over concrete in case of railway bridges.