Admittedly the Vacancy list isn't always updated as fast as it is for those with full access to the Intranet system, but its better than nothing. You also can receive the staff magazine this way as well.
If you work for a TOC, it'll be relatively easy to find on their intranet. One TOC I've worked for provides external access, whilst the other didn't.
Are you an external applicant? If so then there's no point in looking for the internal lists. In my experience, internal vacancies are adapted to ask for your manager's details, so they can arrange to get you a day off to attend an assessment if they're interested.
As I said earlier, go and see your line manager. They are the ones best placed to help you. If you have no luck from them contact ScotRail HR, again, your manager will be able to give you their contact details. Don't ScotRail post a paper copy of the vacancy lists in mess rooms and SOPs?