There seems to be a lot of confusion on the Internet about the course of the old BCR line along Donegall Quay. This is particularly odd as the service wasn't terminated until 1962, so surely there ought to be first-hand sources of its operation.
Per this map from 1922, the BCR line veered left from the East Bridge Street junction and then ran along the Antrim bank of the Lagan, passing under Queen's Bridge in a tunnel. A branch veered right and over the Lagan to Ballymacarret via the Shaky Bridge, replaced in 1976, but let's not complicate things by considering that for now.
On this rather poor 1987 photo of Oxford Street bus station ( a scan from an Ulsterbus publicity shot ) one can trace the line of the railway in this area. Much of the actual railhead was visible in the roadway of the bus station. The perimeter wall survived the closure of the station until demolished as part of the Waterfront development.
According to the RPSI the tunnel was completed in 1879.
The single-track tunnel was fairly short, emerging at the north side of the bridge beside a passenger station and tram interchange. An extension carried the railway on to connect with with the harbour tram system.
It was also quite low: the RT-class locomotives that worked this line through the tunnel had to be specially designed by Beyer-Peacock with reduced overall height.
This photo is described as a scene at the "east end of the Queen's Bridge Tunnel", though I would have said north end. Gasometer in the background. The wall to the right of the image would appear to be that on which these gentlemen are resting as they look east towards the Down side of the river.
After cessation of services in 1962 the tunnel was converted to a pedestrian subway . It linked the north end of Oxford Street bus station with the north-west point of Queen's Bridge. When was this closed? From this challenging prose it seems that it was still open for the receipt of grafitti in 1983. ( See below for BC's updates ).
BC located this excellent shot of the southern approach into the tunnel. Its location can be determined from the Girl's Preparatory School building in the background, still standing today.
BC and TM confirmed that the pedestrian tunnel conversion was T-shaped; one long branch following this line of the tunnel and is now "homaged" by the curious covered walkway parallel with Oxford Street, which protects no-one from the prevailing wind and rain:
The right-angle section crossed at its head from the west side of Anne St / Oxford Street juction ( beside Elliott's costume shop ) to the "dead ground" just across Oxford Street. The cross-subway was abandoned and both entrances filled some time in the mid-2000s. I have not yet discovered the official announcement which should have been posted in the Gazette.