"You may change name but Number]"
There was no such rule, sir. Different railway companies followed different numbering schemes.
Even after independence, before introduction of the 4 digit numbering, trains often had...
more... different numbers on different zones. (This was standardized in the 1960s).
Punjab mail was 5/6 in CR and 37/38 in NR. Pathankot express (present day 11057/58) was 197/198 on CR and 57/58 on NR.
Similarly, before independence too trains changed numbers on different railway networks. Punjab mail was 5/6 on GIPR and 3/4 on NWR (85/86 in later years).
Attached are timetables from 1918 for the Amritsar-Delhi and Delhi- Bombay routes. Note the 4 Bombay mail on the Amritsar section and 6 Punjab mail on the Delhi-Bombay section. Trains often had different numbers and names on railway operated by different companies. Only some trains (like the Frontier mail) had the same name throughout their routes.