I'm wondering, a propos the Indian sub-continent, with its long having had much 5 ft. 6 in.; metre; and narrow (2 ft. 6in. / 2 ft.) gauge. Since a long time back, conversion has happened there at various times, of some narrow-gauge lines, to metre or 5 ft. 6 in. In recent decades, the present-day nation of India has had an ongoing programme of converting most of its metre-gauge trackage, to 5 ft. 6 in. I don't know any specific instances; but it would seem not unlikely that a line or two in that part of the world might have been during its history: first narrow, then metre, and finally 5 ft. 6 in. gauge.
I seem lately to have discovered an authentic instance of the above. The Darjeeling Himalayan Railway was, at its zenith (basically, in the last decades of the British Raj) a several-lines 2 ft. gauge system -- only one of those lines, the famous and surviving one New Jalpaiguri -- Siliguri -- Darjeeling. Another -- slightly longer than the Darjeeling route -- was called the "plains section": ran south-west from Siliguri, across easy low-lying terrain, to terminate at Kishanganj (no other railways connected with there). After independence and partition and the consequent drawing of new borders; the new nation of India lacked a rail link on its own territory, with its extreme north-eastern parts: connected with the rest of the country only by a narrow "corridor" in which was situated Siliguri -- all existing through rail links as at 1947, ran through the territory of hostile East Pakistan (later to become Bangladesh). To remedy this, a link line in Indian territory was rapidly created: from Barsoi on a large metre-gauge network, new construction a little way north to Kishanganj; the Kishanganj -- Siliguri line was acquired and converted from 2 ft. to metre gauge, to join up at Siliguri with the all-metre-gauge system of India's far north-east.
Two or three decades later, Indian 5 ft. 6 in. gauge routes were instituted, linking the rest of the country with its north-east, on the broad gauge, via the Siliguri "corridor". This involved conversion of Barsoi -- Kishanganj -- Siliguri (actually, the brand-new rail hub of New Jalpaiguri, 6 km. south of Siliguri) to 5 ft. 6 in. gauge. So, the 90-odd km. from Kishanganj to the Siliguri / N.J. area started life on the 2 ft. gauge; were converted to metre gauge; and subsequently, to 5 ft. 6 in.
This is perhaps comparable, a little, to the Baltic States situation featured upthread -- regardable likewise, as unusual circumstances: here in the Indian sub-continent, "serial regauging" happening as it did, because of uneasy international matters; even if in the form of geo-political tension and messing-around, rather than (most of the time) full-dress war -- but if the partitioning of India had never happened, probably neither would this particular saga !
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