The scene is when Betty and Bruce (played by Danny Lee) are walking along the KCR track and having a chat about life etc, and there is the distinctive shape of an old KCR railway tunnel. here's a reminder.
Currently there are only two remaining tunnels on the KCR that fit this profile, the first one is the original Beacon Hill tunnel, and the seocnd one is the original single track tunnel at Tai Po Kau. Both tunnels still exist, but only the latter is still in use as a rail tunnel (Beacon Hill Tunnel is now used to carry gas pipes and other conduits from the NT into Kowloon, and to accommodate a second track in Tai Po Kau, the KCR just drilled another tunnel next to the original) but the background we can see doesn't matches anything that can be seen at either tunnels' entrances.
So I pondered that there may have been another tunnel somewhere that fitted the profile. Only today I have had that confirmation courtesy of a photograph from the early 60's shared in FLICKR by our friend Doug. Here is the link: https://flic.kr/p/pjowTd
Look at the background in Doug's photo you can see the tunnel on the right hand side. How long the tunnel was I'm not sure but after a quick look on GoogleEarth it is quite obvious that then hillside in the background behind our two actors is the slope behind the modern day Shatin Hospital and A Kung Kok Shan Road.
Here is another picture of the tunnel courtesy of the CUHK website (it can be seen in the b&w picture at the top of this link: http://www.iso.cuhk.edu.hk/english/publications/newsletter/article.aspx?articleid=54997
The tunnel has been demolished of course, probably due to the lack of space to build a second tunnel for a second track, so they just removed the whole hillside with the tunnel to make the extra space.
Look carefully immediately behind the two actors and you can see what looks to be the start of the reclamation works that filled in the Shatin Sea to make way for Shatin New Town.