In mid-May husband and I took the train from Edinburgh to Inverness that stopped in Stirling. We had no serious issues. It was a nice trip.
If, however, you are traveling with luggage, consider not buying a first-class ticket. (We traveled by train a few times during our trip, but Edinburgh to Inverness was our first, and thinking it would be a nice upgrade, I chose first class.) In a very long train with many carriages, there was only one first class. There was no luggage storage, and a “perk” of first class is curtains, for the purpose (obviously) of blocking out light and, alas, the view! Our travel companions did not close the curtains (one maybe slightly grudgingly), and they put up with our luggage (our hard sided wheeled luggage—slightly larger than standard carry-on dimensions—would not stow in the overhead rack). Fortunately, no one else joined us in our carriage on the journey to Inverness, and we were able to stow our luggage under our table. We were in an arrangement with our two seats on one side of a table facing two other seats.
In subsequent journeys (e.g., day trip from Glasgow to Stirling and back), we saw the wisdom in boarding a train at its origin if traveling with luggage. We had no luggage on our day trip, but on the short journey from Glasgow-Queen Street to Stirling, we noted how quickly seats disappeared, as well as luggage rack space, for those boarding at stations along our way to Stirling. Our train to Stirling from Glasgow would terminate eventually in Inverness. On our return journey from Stirling, the train was packed when it arrived at Stirling. We were able to snag two seats in the same carriage (not together), and some had to stand.
Enjoy your trip!