प्रयागराज एक्सप्रेस - जो भी हो तुम खुदा की कसम; लाजवाब हो

Thanks for tagging me Sir. :)
If you go by the name, then it is a relatively new train. However, there was a MG train that used to run in pretty much the same time slot and with a similar slip coach arrangement (Not much speed-up after gauge conversion).
The oldest train on the route was the Bangalore - Poona mail introduced around 1900 ( The present day 12609/10 and this mail were used to provide a through connection to Vasco and Poona...
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from Madras). It was a painfully slow train taking 2 nights and a day between Bangalore and Poona. Later it was curtailed to Miraj after the gauge conversion and continued running as the Bangalore-Miraj mail until the line was closed for gauge conversion. There were proposals to revive it as a Bangalore-Bombay mail after the conversion, but that never materialized.
In the 1950's, a new Bangalore-Poona express was introduced (infamous for an accident near Belgaum) that was later renamed and restarted as the Mahalaxmi express in 1977 when Miraj-Pune section was transferred to CR. This too doesn't run today.
In 1966, after SCR was formed, they proposed another train between Bangalore and Poona, that would be faster than the mail by half a day. But the gauge conversion between Miraj and Poona had started by then, and instead, the new train was introduced as the Bangalore - Miraj Karnataka express, departing from Bangalore at 8 PM and reaching Miraj at 2 PM the next day. It had slip coaches from to Vasco, that were detached at Londa. This train lost it's name after KK was introduced. Rani Chennamma is pretty much a descendant of this erstwhile Karnataka express.