Hospitals have a duty of care towards suicide prevention on their premises and take as much preventative measure as possible and so do Highways Agency on Motorway Suicide Hot Spots, it would be impossible to stop someone jumping off every building/structure as they will always find an alternative place to attempt an end of life.
I am a suicide survivor and have jumped a few times, thankfully i am on the mend now.
I dont think Railings at Marple Aquaduct will make any difference to suicide prevention whatsoever, it may help reduce the risk of people falling accidentially while taking risks on the non railing side.
Lets not confuse idoits taking risks with people intent on self harm they are two different things.
It would be a shame for the estetics of Marple Aquaduct to be spoilt and possibly not needed, i dont know really.