That's a bugger, but you'd need to look at Scotrail's conditions of travel. I've seen tandems explicitly excluded from travel for some operators (X-Country from memory, but I could be wrong). However, I've taken my tandem on a few South West Trains. SWT are first-come first-served for bikes with specific exclusions into London at peak times. Otherwise it's largely up to the guards' discretion and they're nearly always pretty pragmatic.