The NG alignment would have required a much longer tunnel and the tunneling would be even more difficult as it goes through the heart of the hills. The new BG route skirts around the periphery of the main hilly area and has long straight stretches.
Its just that in this case, the realignment was not purely due to political appeals. IR itself too had benefits from rerouting the track.
Most importantly, there was no clearance to lay the BG track through the sanctuary. The same thing is happening now with the...
more... MG line between Bahraich and Mailani, which is not being converted as it passes through the Dudhwa National park.
But yes, typically IR avoids constructing long bridges whenever possible and prefers longer alignments wherever necessary.