I don't think this is so. The Wikipedia article says, correctly, that the words were first published in 1889. However, it adds that it was "on a train journey from Charing Cross railway station to his home in Honor Oak," and this is not correct. Connell moved to Stondon Park some 25 years after he wrote the words. Street directories and other sources (including the plaque itself) say he lived there from 1915 until his death.

The Dictionary of National Biography says the words were written on "a fifteen-minute train journey from Charing Cross to New Cross". Given that Jim Connell appears in the 1891 census living at 408 New Cross Road, a mere 5 minutes walk from the station, I think it more likely that was where he was living when he wrote the Red Flag (the house survives).

Talking about train journeys from New Cross, there is also Dire Straits' Eastbound Train.