Brunel may have designed the whole thing with a lot of clearance, but elevated passenger platforms have to be sufficiently close to train doors so that passengers don't fall down, so modern platforms should conform to standard standard-gauge British trains and thus likely follow a narrower clearance. British W6 etc. and C1 loading gauges are 2,820 mm wide at platfrom level, UIC loading gauges are 3,150 mm wide, that is, you may need to move back platforms by up to 165 mm. From the point of view of infrastructure, this is a minor problem if Brunel-time platforms only got some light structure added, which can me dismantled easily; but a big problem if concrete or masonry side walls have to be removed and replaced. From a train operator point of view, British-gauge passenger trains on UIC-gauge lines may need telescoping doorsteps to bridge the gap.

_{*Lunatic*, n.
One whose delusions are out of fashion.}