I think you are not from North Side. You may know that Aligarh Jn is situated on Most busiest route i.e New Delhi Howrah line. So Goods train coming from Kanpur Central side and want to go towards Harduaganj, Chandausi, Bareilly needs to cross the NDLS-HWH Mainline which disturbs the traffic on Mainline from both sides. That's why a bridge was needed so that it can pass over the howrah mainline. So if only bridge is to be needed then 3kms approx length of bridge is enough. But here they will make 22 kms long bridge