The two postulates of STR doesn't say that any signal cannot move faster than light. It also doesn't assert that any signal except light cannot have velocity equals to that of light. So at the very beginning of analysis of STR we cannot say anything about the argument of "light having the ultimate(fastest) speed". Infact STR shows that we cannot accelerate a material particle to achieve and exceed the speed of light but we cannot just deny the existence of particles which could be generated similar to photons which at their creation can have speed equals to or greater than $c$ because they have achieved this speed at their birth they are not been accelerated to achieve this speed.
So Suppose there exists a hypothetical signal named schite having velocity=infinity but the speed of light is still same for all the inertial observers "would the events A and B be dissimultaneous in train's frame to validate constancy of speed of light in the Einstein's rail thought experiment"?
Moreover could we use this schite signal to synchronize clocks?
In other words would time dilation and length contraction exist?
I have heard little bit about tachyons. It is said using tachyons we can transmit signals at higher speed than $c$ if (assuming hypothetically) such particles exists can we use them to synchronize clocks?