NOSTALGIA
50 years ago, when the number of trains was MUCH LESS than what it is now,
and the system of UNIQUE TRAIN NUMBERS was not in vogue, the BG trains had single and 2 digit numbers only.
MG trains had 3 digits numbers like 101/102 Madras Egmore-Trivandrum Mails,201/202 Delhi-Ahmedabad Mails etc.Of course there were exceptions like 1/2 Awadh Tirhut Mail (MG). This...
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trend was breached with the numbering of the Howrah-New DelhiRajdhani Express as 101/102.
The train numbers were unique to a ZONE and not the country. Thus there were a series of trains numbered 1/2 like Howrah-Delhi-Kalka Mail (ER), Howrah-Bombay Mail via Nagpur (SER), Madras-Mangalore Mail (SR), Bombay-Amritsar Frontier Mail (WR) to name a few.
The computerisation of the railway reservation system did away with these anomalies with the new system of 4 digit unique numbers for trains.Good that it happened because earlier some trains had 2 sets of numbers used for different laps of their run
13151/13152 Kolkata-Jammu Tawi Express (then Sealdah) was numbered 17/18 (ER) upto Mughalsarai and 51/52 (NR) from there onwards upto Jammu.
13049/13050 Howrah-Amritsar Express (then upto Lucknow) was numbered 15/16 (ER) upto Mughalsarai and 49/50 (NR) from there onwards to Lucknow.
Later on better sense prevailed and ER renumbered these trains as 51/52 and 49/50 respectively to avoid confusion. One reason for this multiple numbering system was NR had its own version of 17/18 which was the Madras-Jammu Janata Express.