Einstein?s derivation of the transformation equations in the Special Theory of Relativity contains multiple leaps of faith in the logic which cannot be justified.
This paper follows on the theme of the comments by Antonio Saraiva.
A symmetrical spacetime model of relative motion is developed in relation to the hyperbola, t? − x? = 1. The model shows the Worldline of P (Inertial Frame coordinates xP, tP) moving symmetrically away from that of Q. If a ray of light leaves P at xP = 0, tP = a-b, is reflected from an event H on Q (xQ = 0, xP = b) and returns to P at xP = 0, tP = a+b, the value tP=a is an overestimate of the time on Ps clock as H occurs. The time overestimate results in an underestimate by P of the velocity of Q relative to P. There is therefore a velocity v = xP/tP = b/a , which is less than the velocity w = xP /(time on Ps clock as H occurs) = b/(<a) derived from a symmetrical model. The former, v, the usual definition, is limited by the equations to less than the speed of light; the latter, w, is not limited. The "twin paradox" is solved.