Did you know that the Ganga & Kaveri rivers are connected? ... by Mysore Varanasi Express - Vishwanath Joshi

I think it is Misleading . The First Digit is given for the below detail .
0xxxx - All Special Trains
1xxxx- All Reserved M/E Trains ( IR is Changing super fast Trains in 2 series)
2xxxx- Superfast Trains
3xxxx-...
more...

Kolkata Local Trains
4xxxx- Chennai Local, Hyderabad MMTS. (47xxx series is alloted to Hyb MMTS)
5xxxx- All Passenger Trains ( in 5 Series the second digit 2 is given for MG/NG Trains and 3 Digit is given for Division (for example If a train number is 52xxx it will indicate that it is MG/NG Train)
6xxxx- All EMU Trains
7xxxx- All DEMU Trains
8xxxx- Left BLANK to implemented in future.
9xxxx- All Mumbai Local Suburban Trains (95 to 99 Alloted to Central Railway local trains)
95xxx Central Railways Main line Fast train Up & Down
96xxx Central Railways Main Line slow up train
97xxx Central Railway Main Line slow Down Train
98xxx Central Railway Harbour line up and down train
99xxx Central Railway Thane- Panvel line up and down train
998xx Central Railway Pune -Lonavala Up and down train
When IR change their train number from 4 to 5 digit they made some error which they are rectifying for some trains .
The second and third digit will be for used for zone and division and ( Except in 9 Series of Mumbai local Trains where the Third digit will be used for slow and Fast train)
The fourth and Fifth digit ( last 2 digit) will be used for that division/zone (which will be 2 and 3 Digit) Train number .
Still IR is working with the 5 digit number to be corrected in future for more detail information.