.......and the popular internet myth about it being the same as Roman chariot wheels is, sadly.....
with a gauge like 4 foot 8 inches, it can only be one of those things. Archaeologists have found similar gauged stone wagonways predating the railways which suggests it was trial and error. Maybe someone first used a narrower gauge but found the wagons tip sideways too easily. Then they proceeded to increased the width as wagon design improved and was resilient enough to stand up to the loading and unloading of stone or coal. Eventually a maximum width of wagon was reached beyond which it required too much extra framing to stop the thing from falling apart.
When the engineers started experimenting with steam engines on rails, they probably thought it was easiest to sell the whole concept to colliery owners if the gauge was kept the same and hence the loading platforms wouldn't have to be changed. Or, maybe the engineers did consider other gauges, but it turned out that the widest wheelset they could practically make without the axle snapping under load, happened to be 4' 8". In use the extra half inch was added after, to the track gauge as a fudge to improve running. So the true gauge of the wheelset is less than the track.
About 6 or 7 years after Stephenson laid the first main railway, Brunel having improved on the engineering chose the wider gauge of 7 foot for his lines. Trouble is he had started later and his system only covered less than a quarter of the UK. Parliament eventually intervened and after an enquiry decreed that Stephenson's 'standard' gauge was to be adopted country wide.
So blame the government all the way back in 1846 who prohibited any new broad gauge lines been laid outside of Brunel's little empire. Eventually the GWR had to convert all it's broadgauge to standard. If only the government had had the foresight to do the opposite. Maybe Stephenson had more friends in parliament, then Brunel did.
Digging down to extend the gauge? Not just flooding, but for single line tunnels particularly, the tunnel is shaped in a oval like er shape. In the days of brick lines tunnels, having an oval shape gave the brick lining strength to resist the surrounding earth. And the oval was planned so it was just wide enough at the bottom for the trains plus clearance. Digging down means you are then lower down against that oval where it starts to narrow further. So if my understanding of civil engineering is right, you either need to change the bottom of the oval for something else and calculate for earth pressure, or broaden the whole tunnel and reline again. But I'm not civils trained, maybe someone who is could say.