This is probably not entirely relevant, but thought I'd mention it FYI as it might be useful at some point...
I've found that if you're booking a Glasgow > Dundee advance ticket, and they're all sold out you can quite often get a Glasgow > Dundee advance through NRE website by selecting 'via Edinburgh' (note: Haymarket doesn't work, for some reason). Which, at £4.10 single with a railcard, strangely enough works out at cheaper than actually going to Edinburgh on the cheapest available Scotrail fare...
Anyway, before I got distracted with that, my point was you can try and buy a cheap advance ticket on this route as well, if it's the one you're going for anyway and aren't tied to the standard off-peak return, which is pretty damn expensive on Glasgow > Dundee!