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China's Belt and Road initiative is forcing more railways in Asia to support "standard gauge" (1435 mm) width, which China and Europe, while the Soviet-era countries use 1520mm. The goal is to be able to run freight from China to Europe.[1] Some track is dual-gauge. There's enough difference between 1435mm and 1520mm to use a 3-rail system. Plain track isn't too hard, but switches are complicated.

[1] https://asiatimes.com/2021/11/chinas-belt-and-road-chugging-...

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This map [2] shows a proposed railway running through Kyrgyzstan to Uzbekistan. Presumably it would continue through Turkmenistan and Iran to Turkey?

[2]: https://www.silkroadbriefing.com/news/2020/12/10/awaiting-on...

> There's enough difference between 1435mm and 1520mm to use a 3-rail system.

Can you provide an example? I've only seen 4-rail dual-gauge systems for those gauges.

Why would you need 4 rails?

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Left wheel uses left rail.

If narrow gauuge, the right wheel uses the innner rail. If wide gauge, the right wheel uses the outer rail.

Because rails are 3 dimensional and has width.

Between 1435mm and 1520mm there's only 85mm space. I am not even sure if you could mount two rail that close.

https://upload.wikimedia.org/wikipedia/commons/f/ff/NYCS_DUD...

3" equals to 76.2mm, which leaves only 8.8mm. No way.

True, I didn't think about how close these gauges were, and was assuming adequate space for the inner and outer rail.

  These gauges cannot make 3-rail dual gauge with Russian gauge.

    1,676 mm (5 ft 6 in) Indian gauge
    1,668 mm (5 ft 5+21⁄32 in) Iberian gauge
    1,600 mm (5 ft 3 in) Irish gauge
    1,435 mm (4 ft 8+1⁄2 in) standard gauge

https://en.wikipedia.org/wiki/5_ft_and_1520_mm_gauge_railway...

I got that info reading about a dual-gauge solution for China's Belt and Road. But it was a political piece on the "Akhaura-Sylhet dual gauge project" in Bangladesh. Turns out that India, Pakistan, western Bangladesh, and Sri Lanka (also, SF's BART) use 1,676 mm gauge. There's enough space between that and 1,435 mm standard gauge for 3-rail. Not sure about 4-rail.

See my other comment[0] on this.

4-rail is a viable solution, but, unsurprisingly, it costs more than a 3-rail or sometimes a two separate 2-rail lines.

[0] https://news.ycombinator.com/item?id=31880979

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