Cornwall Street now runs from Waterloo Road, all the way up to the Chak On Estate near Tai Wo Ping, however, originally it only ran as far as the lower section of Beacon Hill Road (which for some reason is completely disconnected from the upper part where Betty used to live!) - which is basically where the closest traffic bollard is in the above screen grabs. Even by 1973, just a few years before this film was made, the western end of Cornwall Street had been laid out but the Govt had yet to make the two ends meet across the railway track near the Beacon Hill Tunnel - this is essentially the section we can see above. Look closely and you can see where the bridge over the track is/was.
Of course the bridge is still there even though the track has now been moved, but the old trackway still exists and so the bridge still needs to be there. Anyway, here is a modern comparison and yo can see how much has actually changed. That's where Beacon Hill Road (lower section) starts on the right and marks the initial extent of Cornwall Street before its extension.