What's your minimum radius for your spectrum HHP-8's?
The HO scale HHP-8 performs best on 457.2 millimeter radius curves or greater.
I have run mine, without cars, on 18r.
Hmm, went paging through pdf of 2016 print catalog and didn't see it in there: print catalog usually gives radius info. E60CP and GG1 were in there on pages 68-71, but not the modern job.
And its listing in Bachmann's web store doesn't appear to have that info.