For a RailFan, watching MPS skips are far more interesting than watching F1 race - Prince Maan

Janta express trains were introduced after independence. There were no Janta express trains before independence.
And regarding the 'proof' I provided, You mentioned a 'pre-independence' train and I provided a 'pre-independence' timetable. I guess there is no need to dismiss it as irrelevant.
There was a 'Delhi' express that ran from Colaba terminus to Delhi (Short terminated at BCT from 1930), which started in 1911. You can this in the timetable from 1937 - click here . At the time, the Frontier...
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mail and the Delhi express trains were the only trains to Delhi operated by BB&CIR. From December 30th, 1945, it was extended to Lahore via Bhatinda and renamed as the 'Lahore' express. It was this train that was restarted as the FZR Janata express - click here . Also, after independence, the train was restarted as 23/24 BCT-Delhi express (It was running between these two stations in 1961). Only later was it extended to FZR.
Before the Frontier mail in 1928, there were no regular trains from BB&CIR to Lahore or Peshawar. The only trains between Bombay and Lahore were the GIP Peshawar express (Also known as the Punjab mail - present day 11057) going via Ambala, Amritsar and the GIP Punjab mail going via Bhatinda, FZR, Kasur. BB&CI only operated postal specials between Bombay and Delhi, which connected with the GIPR trains at Delhi. Their main route to Lahore was Bombay-Ahmedabad by BG, Ahmedabad-Hyderabad (Pakistan) by MG, and further to Lahore by BG.
The Frontier mail in 1928, was the first 'regular' BG train operated between Bombay and Lahore by the BB&CIR. The Frontier mail did take the route via Bhatinda initially, but it was diverted via Ambala,Amritsar from September 1st, 1930.
Many trains did go between Delhi and Lahore by the route you mentioned, but none of them were named as 'Punjab express'. And 19023's predecessor definitely did not run between Bombay and Lahore between 1912-1937. The extension to Lahore only came shortly before independence and the train ran to Lahore for barely over a year.