For the last 30yrs it was not running in BVRM route and no train is diverted from BVRM route to mainline as far as my train experience is concerned in past 30yrs. Any seniors like SRG Circar exp can confirm. Request you not to take youtubers seriously in quoting proofs as most of them are youngsters below 30yrs and for them they are fans of trains, buses and make videos without proper knowledge or check the past data. Tatanagar Chennai exp was introduced in 1965-66 period via mainline and ever ran via BVRM branch kine.later link train is attached from Ranchi further extended to Bokaro in 1980s period. The train is extended to Ernakulam from Chennai later and finally Bokaro portion is extended to Dhanbad. This was conveyed to me by my father in a discussion now.