This is terminology that has been interpreted thousands of times by the courts. It is readily understandable to everyone.
The day that this bill comes into effect, the railway will have an obligation to meet the standard. So it's easy for the railway, given the wide range of existing jurisprudence, to know what constitutes an "unreasonable noise". It will also be easy for the agency, should there ever be a complaint, to determine whether or not the railway met its obligation, because the jurisprudence establishes what is meant by unreasonable noise.
As for the other expression, it's not that it would be impossible for the agency to decide on whether or not a complaint was warranted if we use the words "as little noise as possible", it's just that there would be a little bit of uncertainty for a period of perhaps one, two or three years, namely, the time it will take for the agency to establish jurisprudence on the significance of the expression "as little noise as possible".
So that would create certainty which did not exist in the previous bill.