The BL 18-inch railway howitzer (formally Ordnance BL 18-inch Mk I howitzer on truck, railway) was a British railway gun developed during World War I. Part of the progression of ever-larger howitzers on the Western Front, it did not enter service until 1920.

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n 1940 there were concerns that an enemy invasion was imminent, crossing the English Channel from France. Three heavy rail-mounted guns were deployed on the Elham Valley Railway line in Kent. The railway route followed a meandering course, enabling the guns to be trained by moving them along the line to a suitable location. The wooded landscape also gave cover for the guns.

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The gun was first fired, for calibration tests, on the morning of 13 February 1941, when the equipment was towed to a stretch of track near the Black Robin public house, Kingston. Several rounds were fired out into mid-channel, the results of which were sighted and marked by observation posts on the cliffs at Dover. In the Kingston and Barham area villagers were warned to open doors and windows, but the blasts were so severe that in many cases houses were damaged.