In this series, the main character has flashbacks to a time in his childhood which haunts him, and this memory envolves him walking into a spooky looking abandoned railway tunnel.
I have done a bit of digging and come up with an answer.
The series is stated to be almost entirely Filmed in Edinburgh, with some bits in Glasgow. But this abandoned tunnel doesn't appear to coincide with any in the Edinburgh area. There are many more contenders in Glasgow, but one thing that stumped me was that in the tunnel scene in the series, there are what appears to be rails still in place (mothballed to use a better term) within the tunnel itself, there also appears to be a bridge seen to span the line shortly before the tunnel entrance. I thought it could be the Boughton Brake Tunnel on the Bevercoats Branch which used to serve industrial trains from Bevercoats Colliery in Nottinghamshire, simply due to the fact that the rails are still in place and the proximity of a road bridge close to the tunnel entrance, but the tunnel face is of an entirely different design. I then tried searching for tunnels on heritage lines that may have just been "foliaged" for effect, but nothing came up which resembled the tunnel in the series.
For a while I scaned UKrailmap looking for old lines with tunnels and the track still in place, but gave up a little while ago.
Anyway, today after actually just coming back from a holiday in Scotland, I went on railmap to check out potential railway walks for my next visit and stumbled on the town of Greenock west of Glasgow. I noticed a few abandoned lines in the area with one (The Greenock Princes Pier Junction Railway) appearing to have a remarkable amount of Tunnels on a very short stretch of line through the town. Anyway, I typed it into Google and what should come up, a thumbnail of a Youtuber called James Perry (image below) walking into a tunnel which is a dead ringer for the one in the series. After further image searches, I confirmed that the rails where still there as well. I am 99% percent sure this is the tunnel used in the flashback scene throughout the Crime TV series.
It doesn't seem to have a name but it is located to the west of Mearns Street (which is the road bridge we can see in the scene), I have included a rather crude mockup image below of the location incase anyone wanted to explore.
Anyway, I can sleep now without that niggling at me. Hope the bloke who asked the question sees this post.
Cheers.