# Why did the USSR switch from 1524 mm gauge to 1520 mm gauge in the late 1960's?

This article mentions that Russia started with a 1,524 mm rail gauge, and in the sixties and seventies, switched to a 1,520 mm rail gauge with tighter tolerances.

Why do this? If you want tighter tolerances for faster train speeds, why not stay with 1,524 mm and just tighten the tolerances? The article and related ones also mention that the tolerances are still greater than 4 mm, such that 1524 and 1520 are virtually synonymous and you don't even have to switch cars or bogeys like a real break-of-gauge.

So...why did Brezhnev make this change? It seems a lot less work to just stay with 1,524 mm and tighten tolerances if you want.

• What are tolerances? Sep 4, 2015 at 12:53
• @NeMo tolerances are the margins for engineering purposes. It's impossible to build a gauge that's perfectly 1524 mm wide. So the engineering blueprints say something like 15244 mm ± 15 mm. The plus or minus 15 mm is the tolerance. You can specify smaller tolerances which will enable higher safe speeds. Sep 4, 2015 at 20:39
• AFAIK, tolerances were changed at the same time (they are, and have always been, asymmetric). The point was such that all track good under the old standard would remain good under the new one (with upper values being gradually reduced later on). What changed was the new target value for track maintainers and builders to meet in view of the new formulae for track quality assessment. As to Brezhnev, I doubt about his ever having heard about track gauge being changed.
– ach
Sep 6, 2015 at 0:10

The Wikipedia article you refer to gives a reference which says that this change "increased speed and stability". Which can be explained, of course. Suppose you have two gauges, 1524 and 1520 with the same tolerance, say 4. This (very roughly) means that the first one will really have width from 1520 to 1528, and the second from 1516 to 1524. ("Roughly speaking" because really there will be some probability distribution of the width, but we can consider a simplified model. In any case, 1524 is only the AVERAGE distance. The true distance varies. Hopefully within the tolerance limit.). Then probably 1528 is too wide for "speed and stability" of the existing rolling stock, and again it is easy to explain why. The wheel has inner rim which is on the inside side of the rail. This inner rim is needed for stability. If the distance between these inner rims is much smaller than the gauge, there will be an instability.

Shortly speaking the change can be justified exactly in the case when it is difficult to tighten the tolerances.

On my opinion, this is a purely engineering question, and I doubt that this was "Brezhnev's decision". Though of course he could stamp this decision, as a very important one for performance of the railroad system. And the question is purely technical, hardly belongs to "History".

This was done to improve stability of the existing rolling stock. If they just changed the tolerances, they would need both new stock and new rails for the benefits to have force. With changing the standard of the rails they could make use of the improved stability using existing rolling stock on the newer rail lines.

• Do you have a source for any of this? It's hard for me to believe because it seems way easier to modify train cars or train bogeys than to modify hundreds of thousands of train track. Sep 4, 2015 at 20:42
• If they modified only the cars they could end up with the cars not able to roll the current rails because the tolerancies of the existing rails were wider. Sep 4, 2015 at 21:47