As Heather said! - "British" Railways, since Isambard was a boy were clever enough to realise it was pointless re-inventing the wheel
and bought in all of their standard rolled steel sections - channels, Tees, equal / unequal angles etc. and how they were orientated when they were symmetrical like the solebar channel was just down to how they were cut and loaded onto the drilling jigs. The railways were obviously a massive user of steel - so much so they were part of the committee's set up to standardise sections and shapes from the pre- British Standard days....