What would happen if some signal could move faster than light? 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?
 A: 
The two postulates of STR doesn't say that any signal cannot move faster than light.

This superficial appearance is deceiving. After all, the postulates involve notions such as inertial frame and speed; therefore they require and presume definitions of      


*

*how to measure whether or not two participants (such as a "sender $A$" and a "receiver $B$") had been "at rest to each other" throughout a trial under consideration; and, for two participants who were at rest to each other:

*how to measure "distance between" them, and how to measure "ping duration" (of any one; from having stated a signal indication until having observed that the other had observed the signal), to be consistens with Einstein's prescription: "In accordance with experience we shall assume that the magnitude 2 distance / ping duration is a universal constant (c)".
Correspondingly, here c is of course understood as (magnitude of) signal front velocity (instead of "phase velocity", or "group velocity").
In summary: the detailed definitions of the notions and quantities appearing in the famous formulation of the postulates of STR imply that any "speed" is determined in reference to unambiguous "signal speed".     
And applied to the question title asked above:
it is of course an absurd proposition that some signal should "move faster" (from some particular sender, to some particular receiver) than any signal exchanged between this particular pair of participants.
A: we cant track a signal moving with a velocity more than C.we can say that it will not interfere with the material matter.it can pass through any matter with out making a interaction with it as it will pass through the matter as of its momentum hf/c2*infinity in this instance which is very high. even though it passes through it the body will be perfectly elastic and will retain its orginal shape with no deformation.
