While decomposing the impulse train using the Fourier series the number of the samples are infinite hence representing a infinite length of train.
But while studying the Nyquist Criteria in the frequency domain it is shown the expression to have $m$ samples where $-\infty<m<\infty$
While studying the concept of the Nyquist Criteria in the ISI model where for the system to be interference free the value of the sampling instant must be given as following:
For more reference here. I cannot understand that the signal is having infinite duration and infnite bandwidth accordingly is it even possible.