the middle one is fulfilling the duty of care to both the workers and the road users.
other two, it is much much cheaper and safer to do the work offsite and bring it onsite. working on the ground is much cheaper and easier than working at height. In terms of the rail bridge, railway line would’ve probably only been shut for a week max to put the bridge in and if you were building in situ it would have been more like a year.
with the power line, how else would they build it without access? if they’re building the access then not building it with proper drainage would potentially mean the works couldn’t be completed due to programme due to bad weather and would’ve held up other parts of what is likely a much bigger project.
cabins are to fit in with duty of care to workers – need to provide so much canteen/drying/welfare/wc etc space per employee.