5. Distance between rails
Normal track gauge is measured as the distance between rails. They are measured 14 mm below the top edge of the rails (Rail top). The standard value is 1435 mm, but it is allowed to be down to 1430 mm and up to 1470 mm before it needs to be adjusted. In simpler terms, the center distance between rails is 1.5 meters. The railway wheel has a flat roller path and on the inside of the rails the wheel has a flange. The transition between the flat roller path and the flange is not sharp, but is curved with a slow transition between roller path and flange. The finesse of this is that if the railway vehicle tends to move e.g. to the right of the track, the wheels on the right side with their curved surface will start to "climb" up on the rails. There on the curved surface, the circumference of the wheel is larger and the wheel thus automatically steers the railway vehicle back towards the middle between the rails. This function is independent of the track gauge. If you increase the track width while maintaining the distance between the wheels, the impact effect will not be so strong. There is a discussion about increasing the nominal dimension 1435 to 1437 millimeters. The advantage would then be that at high speeds the so-called sinus movement would be smaller on long straight stretches of the track.
Risc to turn over. Railway vehicles have a so-called load profile. It indicates e.g. the maximum height and width of the vehicle. In Sweden, we have a load profile: A 3,40 m wide and 4,65 m high, B 3,40 m wide and 4,30 m high and C 3,60 m wide and 4,83 m high. Or simpler, generalized; 3,5 m width and 4,5 m height. To get a low center of gravity, the ratio of height over width should be as small as possible 4,5 / 3,5 about 1,3 (This is a simplified way of looking at the matter, of course knowledge of the actual central of gravity above the base surface is preferable, but it is not readily available). In order to have a stable function, the support surface must be as wide as possible, the ratio of vehicle width relative to track gauge must be as small as possible 3,5 / 1,5 gives approximately 2,3. The product between the relative height of the center of gravity 1,3 and the relative value of the support surface 2,3 then becomes a typical roll risk value of 3. Let us compare with a passenger car. We then choose an SUV that is significantly more prone to overturning than a sports car. Values for the Volvo XC90, height width ratio = relative center of gravity height 0,92 and the relative support value of the support surface close to 1 (the wheels placed as far to the side as possible). The inherent risc to overturn is three times larger for the train. If we made the comparison with a race car, the difference would obviously be even worse for the railway vehicle. Vehicles that stay on the "right keel" in an accident cause considerably less damage to their passengers, so low center of gravity and wide base are always preferable.
The reason for the large load profile is of course the desire for the opportunity to take large loads. The gauge was early so-called narrow gauge (narrow gauge is less expensive to build). As the state took over the railway in the Sweden, we now have a normal width of 1.5 m (Exception of Stockholm's local traffic operated Roslagsbana with just under 0.9 m between rails). Changing the existing railway network to something more technically sensible is not economically reasonable. Not only tracks but also the fleet must be changed.
A larger track gauge is safer. For a completely separate new railway, which requires vehicles with extra wheel axles for the gear unit without moving parts, the situation is different. Here we have the chance to develop technology that is significantly safer than what we have today. As for the car, we can choose a track gauge that corresponds more close to the vehicle width, e.g. for railway vehicles 3 meters between rails instead of 1,5 meters. The risk of overturning is minimized. We can choose an optimal load profile. And not least valuable, the technology for suspension will be simpler, cheaper and more robust. Finally, we can have a larger cant elevation in horizontal curves. We can easily drive on a bridge over a motorway, probably without active tilting. Note in particular that on Jo-Jo-han all trains run on the main track constantly with a very small speed variation, only +-5 km/h. We get a smooth flow and the track can be optimized for the speed of 245 km/h. Optimal and unbeatable comfort for the traveler.