For the curious, Russian gauge is/was 5 ft (1,524 mm) and standard gauge is 4 ft 8 1⁄2 in (1,435 mm). Soviets eventually redefined it to a round metric number (1,520 mm), but that's a very small difference. Track width varies by around a centimeter normally.
Russian and Soviet rail profile tend to be bigger (taller by maybe an inch) but that doesn't really impact the trains aside from transitioning from one to the other.
Interestingly, 3.5 in happens to be almost the ideal change. The bottom of a normal rail profile is a bit over 6 in wide, so the new spike hole is right between the two old holes. If the difference was any larger or smaller, the new spike wouldn't hold as well.
Russian and Soviet rail profile tend to be bigger (taller by maybe an inch) but that doesn't really impact the trains aside from transitioning from one to the other.
Interestingly, 3.5 in happens to be almost the ideal change. The bottom of a normal rail profile is a bit over 6 in wide, so the new spike hole is right between the two old holes. If the difference was any larger or smaller, the new spike wouldn't hold as well.