Earlier the zones were quite large. ER was probably the largest at one time, though it is difficult to come up with an exact number.
When it was formed, ER contained all of the present-day ER, ECR south of the Ganga, ECoR, SECR and SER. It was quite large.
SR contained all of present-day SR, SWR and BZA/GTL/GNT divisions of SCR.
NER contained all...
more... of present day NER, ECR north of the Ganga and all of NFR, extending from Mathura till Dibrugarh! It was probably the largest in terms of track length.