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American railroads were generally built later than British ones, and the lack of the same level of already existing built environment made railroads able to construct their lines to a wider loading gauge in North America. So, British railways are generally older but do the job well enough that making modern trains to fit the older lines is what's preferred over renovating the lines to increase the loading gauge. As well, there's little need for huge trains in the UK, as the railways' primary purpose is passenger transport, as opposed to freight like in the US which benefits hugely from massive increases in scale, making many renovations economical.
