Not sure if I understand your suggestion correctly. I made a new layer, selected the shape of my station and filled it with 100% opacity in grey, then I took the many layers that make up the station, merged them into one layer, placed it above my new layer and set it to Overlay/Hard Light. But all that did was that the entire station got brighter or darker depending on the color I used for the new layer. Maybe that's the point of this procedure, but if it's not please elaborate
Also, do you have a suggestion on how to efficiently create the pipes and the grid?