Yes. The Castle Class locomotive did operate on the Great
Western Railway. It also operated in the Western Region after the
"Big Four"(GWR, SR, LNER, LMS) became British Railways in 1947.
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It is: 2721 = MMDCCXXI
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GWR Records ended in 1991.
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Brentford railway station - GWR - was created in 1860.