I don't know how good the spotting is, but
Mossend is the first place that comes to mind.
Edit: And Grangemouth is the second.
I believe Mossend is pretty tricky to get a good view of (per a friend who lives up that way - never visited myself). But it's definitely the first place to spring to my mind too, so maybe somewhere on the routes in/out of there.
Typically with freight I think it tends to come alive more at night too.
It may be a case of looking at where you're going and seeing what freights might be in those areas - they're less frequent, but you do get some decent ones, e.g. on the lines up to Inverness/Aberdeen. There's DRS Tesco intermodals (68s/66s), Colas 60s on the cement trains, DB calcium carbonate slurry tanks, the GBRf aluminium(?) trains that work up the West Highland line etc.