Figure 2a. The rail (bold vertical line) as observed in its own inertial frame of reference. The rail is identified with the -axis of respective coordinate system. Each line segment (both vertical and horizontal) determines the unitary proper length, defined as “unit length”

From

Why and How Special Relativity Fails in the Second Spatial Dimension? An Introduction to the Preferred Frame Theory

Maciej Rybicki

International Journal of Physics. 2024, 12(4), 175-195 doi:10.12691/ijp-12-4-5