In the Lorentzian spacetime: 
In the model of Euclidean Reality (FER):

The spacetime interval is equal to the distance in the Lorentzian spacetime.
This distance can be:
 positive
 equal to zero
 imaginary (square of the distance can be negative)

In the figure below is presented observation of object moving along the black trajectory, by observeres moving
along trajectories green and blue (both observers have common spaceaxis).
In the FER model the idea of the spacetime interval is reduced to the cases of mutual observations.
The spacetime interval is here equal to the proper time of observed object
Its properties:
 can be positive
 can't be equal to zero (if there no exists any non zero factors in the interval formula)
 can't be imaginary (square of the distance can not be negative)

CONCLUSIONS:

In the Lorentzian spacetime: 
In the model of Euclidean Reality (FER):

 The spacetime interval can be imaginary then tachions
(hypothetical superluminal particles  for which the space time interval is imaginary) can exist
 The spacetime interval equal to zero is equivalent to existence singularities.
It means that, for instance, any object, having nonzero mass, can not reach the speed
of light (for motion with the speed of light the spacetime interval is equal to zero)

 The spacetime interval can NOT be imaginary then tachions can NOT exist
 The spacetime interval equal to zero is equivalent to moving of an object along trajectory being
perpendicular to the trajectory of an observer. It does not mean that any singularity exists here. The object can be then
accelerated to the trajectory perpendicular to trajectory of an observer (moving along this trajectory is interpreted as moving with the speed of light),
however the observation of process of the acceleration will last to infinity. For instance  a rocket can be accelerate to such trajectory
but astronauts will be able to observe themselves accelerating to the speed of light, even many years after their coming back from the interstellar journey
