Navigating Purchase and Train Schedule Stirling (STG) to Leuchars (LEU) to get to St. Andrews

Scotrail is what you should use - they run the trains and sell the tickets

https://www.scotrail.co.uk/

I don't know why the through ticket via Dundee isn't being sold by Scotrail.

Just because Google creates a journey doesn't mean you can buy it. Trains don't sell out - certain ticket types might. If you really want to travel via Dundee at that time you can buy two tickets: Stirling > Dundee; Dundee > Leuchars (then bus to St Andrews).

There is a concept within railway ticketing in the UK called "permitted mileage" - the same as in airline fares where only certain routes are allowed.
In essence one of the tenets of the rule is that it does not allow you to double back on your route. Because your proposed route would mean that you would go past the Tay Bridge into Dundee and then come back out the same way to go over the bridge to Leuchars it is not a valid route. Thus, daft as it may sound, the through fare takes you on either a Perth/Ladybank route or a via Haymarket route.
I know you will say that via Haymarket involves doubling back- that is true, and that one is harder to explain.

Anyway your proposed route can be booked as two tickets-
Stirling to Dundee;
Dundee to Leuchars

The #92A is run by Moffat and Williamson, not the main local operator, Stagecoach, who have a #99 to St Andrews Bus Station from Leuchars just a minute or two earlier.
There is a through fare from Dundee to St Andrews including the bus from Leuchars, but in this case paying separately is the cheaper option.
Traveline Scotland to me shows buses all day, but for whatever reason takes you from Stirling to Dundee on the Megabus/Citylink. Again you can force it to give 'your' route by dividing it up.
As has been said Google maps is an unusual way, to me at least, to plan a route, and expect to get bookable results.
But Traveline Scotland, even if it gives the right route, rarely (as in this case) gives the right fares. As I have had many times before traveline scotland gives about double the right fare.

I appreciate the responses from the last 2 contributors. Now it makes a lot more sense. I looked at the first leg, Stirling to Dundee, late last night, and there was only one ticket left at £19.20. I guess I would have had to buy one at £19.20 and one at £35.20. I will most likely take the route that involves Ladybank. But in general, how much time should I allow between train transfers? Can trains run late?
Thank you also for addressing the bus companies, as I forgot they were also privatized. I will try to get all tickets such that I don't have to wait an hour at a bus stop nor catch a bus with a lot of stop and go's. Is Rome2Rio an accurate site for comparing bus schedules and travel times? Is there a better one other than going to each bus company website and looking at their time tables?
In regards to websites that list routes, I found out that there are two websites, Traveline.info and TravelineScotland.com. TravelineScotland.com appeared much more comprehensive. Maybe my browser configuration affected the way information was presented in Traveline.info.

Rome2Rio is awful when it comes to exact schedules. Traveline.info is the English version of Traveline. Traveline Scotland is the one you want. Apart from dodgy fares that one is good and always up to date on schedules.

I'm seeing an advance fare of £9.60 each Stirling to Dundee (so tied to that train) and £6.40 each anytime day single Dundee to Leuchars (so good for any train). The standard rule is to allow an 8 minute connection time between trains. If the first train runs late your ticket is automatically good for the next train (even for an advance ticket)- no need to exchange it or anything.
For the via Ladybank route I am seeing an advance through fare of £9.60 each with 20 minutes to switch trains at Perth and 20 minutes at Ladybank. Ladybank is a simple 2 track station so that switch especially is easy.
All the conductors have i pads linked to live train running data, so they can easily verify if a train was late.
If you go the Dundee route (but not the Ladybank route) you can book to 'St Andrews Bus', rather than Leuchars. That includes the stagecoach #99 between Leuchars and St Andrews (that route runs every 10 minutes). From the Bus Station it is a short walk to Murray Park. I don't think it is valid on the #92A Moffat and Williamson's bus from Leuchars, but can't confirm that either way.
The thing is Stagecoach is so frequent that is the default route- no one normally thinks about using the M and W bus.

an excellent, comprehensive site for bus timetables is https://bustimes.org/

Another option, if you prefer the route via Dundee, is to take bus 99 from Dundee to St Andrews. It means you won't have to change in Leuchars.

I appreciate all the information thus far. I bought my tickets from Stirling to Dundee (specific train, non-refundable, £19.20) then Dundee to Leuchars (anytime.£12.80) even though that route had an added cost than going through Ladybank since there seems to be more options. I was under the assumption that the two together rail card doesn't apply to non-refundable advance tickets because when I tested it out, the price was the same. But, maybe I am dazed and confused because testing it again, it showed a discount with two together rail card. Can anyone confirm that the rail card applies to non-refundable advance tickets? On another note, I did go ahead and buy the two together rail card. I think I would not be able to go back and get the discount on tickets I already purchase, right? I am pretty sure I can't for STG-DUN, but maybe for DUN-LEU? Is it a hassle to get a refund, then repurchase the ticket if it ends up being less expensive? My travel itinerary is EDB-STG-LEU-St. Andrews-INV-EDB, so I still have a lot more ticket purchasing to do. I really do appreciate the input. I have spent hours researching and testing, but I find this forum a extremely good way of getting the answers I need.

...More questions: LEU-INV: I know isn31c mentions 8 mins train transfer time is standard, but is 6 mins in Aberdeen doable? I do tend to get turned around. I'm looking at the 10:27 arr 14:11. Will it be scenic? I have heard the route via Perth, though longer, is scenic and what better way to enter the Highlands? I will be trying to take the direct train INV-EDB, but I like the idea of scenery in the beginning part of my trip. Also, do you get a better view of the scenery in 1st class?

2T discount is available on all ticket types a visitor is ever likely to use (there are a handful of very exotic exceptions), as long as you travel after 0930 on a weekday, or at any time on a weekend, and can be used in 1st or standard class.

The tickets you have purchased STG-DUN are non refundable. DUN-LEU in theory you could refund and get new ones with 2T discount- but as it is your choice to refund (not failure to provide a train service) there is an admin fee of £10 which wipes out the benefit and more.

6 minutes is do-able. Moreover this is an exception and 5 minutes is the official Minimum Connecting Time. Journey planners give it as a valid through route so that is OK. But to be very clear the official policy is it is only a connection IF the train from Leuchars is on time. The Inverness connection might be held if the Leuchars train is 5 or 6 down, but not any later.
However looking back at last Friday the Leuchars train was 16 minutes late at Aberdeen (arriving on Platform 4), but the Inverness was held for that train on Platform 6N (N for North, as the very long platform continues as 6S [outh]) and departed 14 minutes late.
Beyond the footbridge is 6N.
How long they would have held it for is a good question- that is a long hold by UK standards.

Booking several weeks ahead 2 people on 2T will get that through fare for a total of £27.90 saving £14.50.

Both routes are scenic in their own right- in different ways. Via Aberdeen is more so south than north of ABZ in my opinion. From Leuchars that is the best and cheapest route anyway. Between Leuchars and ABZ try to sit on the right hand side of the train for the best views. North of ABZ I don't think it matters much.

This is a plan of Aberdeen station showing that it is a flat (no steps) change- https://images.nationalrail.co.uk/e8xgegruud3g/1p3QgzWWQhYmeMn45G8OTu/5a5209fc590c2d7084dfb236f1ea3c98/Aberdeen_station_map.jpg

Getting the INV-EDB as advance fares several weeks ahead on 2T, at 1050, should cost a total of £25.30 (saving £13.10).

Your initial EDB-STG sector has no advance fares- so either buy in advance or on the day- 2T for 2 people £14.20 (saving £7.40) so the 2T will still have paid for itself.

All fares quoted are standard class.

No the views in 1st class are the same as standard- same size windows- just 1st may be less crowded.

Re LEU-INV: Interesting about the 16 minute delay and INV train held. Also, thank you for letting me know I can look up past train services. Can you show me how to look up a past train service?

So now, I'm debating whether I should hold my breath and hope the LEU-ABZ train is not late or just play it safe and take the 9:32, arr 13:25 with 2 train stops even though I won't have a leisurely morning. I have to be in Inverness before 16:00 to pick up a rental car on Saturday or else I won't be able to get it 'til Monday when they open. I guess if I want two different scenery routes, I could also buy 2 tickets- LEU-ABZ on the 9:32, arriving 11:05 and wait about an hour to catch the 11:57, but I don't know the rail's obligation to me if I don't have a ticket with a connection. Comments?

Thanks also for the map link of the train station. I didn't know that existed, but now I see how I can look up other station maps.

The 0932 to Inverness, 2 changes, is actually taking you through Perth and the Highland Main Line rather than through ABZ.

But the 0932 does continue to ABZ as you state, so to force that route you would have to split it into two tickets. If the 0932 was 50 odd minutes late (or cancelled) then Scotrail will know they have a problem.
If it was between 30 and 60 late then, through delay repay, you get half the cost of your affected ticket back. Over 60 late then you get full ticket cost back.

The railway obligation in theory is that, if your delay is over an hour they are supposed to provide other transport- taxi/bus etc. If there was such severe service disruption, ask the train conductor to phone ahead to ABZ showing him/her both tickets (which proves you had allowed more than ample time for a connection). Chances are you will not be alone in that predicament. Either local staff or Scotrail Control centrally would then decide what to do- hold the Inv train or make arrangements for road transport.
Tell them you were ticketed that way as you had a meeting in Aberdeen in the 45 minutes or so connection time (or similar) to explain the slower route.

This is how you check both live and the previous week's train running (and what unit has formed the train)- https://www.realtimetrains.co.uk/

So for instance, last Friday, I can see that the delay to the train from Leuchars happened on the Tay Bridge (probably a signal or points failure).

On Thursday that train was even later (arriving at 1210, also delayed at the Tay Bridge) and the Inv connection was not held, on Monday, Tuesday and Wednesday it was a few minutes late and the Inverness connection was held.
So it looks as if 15 minutes is as long as they will hold the Inverness connection for.

Thank you for the link. Now I see how this works. What is a signal or point failure? Can that happen anytime of the week? I'm now thinking since I'll be traveling on a Saturday, my risks might be lower because there's less rail traffic, if that has anything to do with signal or point failure?

The railway infrastructure can have faults at anytime. It is intriguing to happen to the same train at the same location two days on the row.

Points are switches in US speak- where a train can change tracks.
If a switch fails to work properly then it is a "points failure", which has to be rectified before the train can proceed. Either the switch has failed to work, or it has not locked into place.
Signals control access to sections of track to keep distance between trains. I don't know how they work in the states, but over here if there is any kind of a fault signals will change automatically to red, to stop traffic (fail safe mode). Before the train can be released into the next section the cause of the fault has to be determined. There is other lineside equipment to automatically stop a train if the driver tries to go through a red signal.
Often our railways are more sophisticated than US railroads.

The Talgo derailment on the Cascades Service in WA several years ago, while partly due to overspeed, simply could not have happened in the UK as a combination of systems would have over ridden the driver errors.

This is all getting way more technical than the average rider needs to think about. One of the things about Real Time Trains (above) is that it has way too much information, and confuses the average train rider more than it assists.
I use it all the time when I am travelling, often I see a problem building long before it is announced to the public. And at big stations, I usually know the track number of my train long before it is announced.

As a detailed person, I really appreciate all the information. I might wait a bit before I buy my ticket LEU-INV to see if they really fix the issue. Thank you so much.

I've been waiting for price drop with advanced fares for 30 Sep, INV-EDB. Any reason why they haven't dropped yet? It's less than 8 weeks away.

There is no obvious reason, not are advance fares out for the Monday and Tuesday following, either.
Don't seem to be any engineering announcements but I'm on terrible Wi-Fi currently on HS1. Will try to look again later when on Avanti.

EDIT- Confirmed no current scheduled engineering work that day.

Euston's Wifi is 1000% better!!

@lyntobrow- the cheaper Inverness to Edinburgh advance fares are now issued for 30 September. Most trains are now £19.50 each before railcard, or £25.30 for the two of you with 2T railcard discount.

Thanks for checking, as I haven't checked, being out of town for awhile. The route says engineering work might affect the schedule and to check back later. What exactly does this mean regarding my obligation to stick with this ticket? Does it mean I'm obligated to take whatever route and schedule that the train changes to instead of having the option to take a different schedule and route?

Well it does mean that, yes.

But at the end of the day there are only two routes the train can take- direct to Perth or round through Aberdeen. The fact that they have released tickets at under 8 weeks out on the direct route says they are confident of no engineering work.
Both parts of the via ABZ route have the same warning.
If they weren't tickets would have been held off sale.
It looks like just covering their own back.
And neither LNER or Cal Sleeper believe that there will be engineering work either. So if it happens that there is engineering then a lot of people will be taken by surprise.

At this stage it seems very unlikely that there would be a line closure with bus replacements. It might happen that the line south of Perth was closed, so they have to divert via Fife which adds on half an hour if that.

Put it this way, if they close the Highland Main Line unexpectedly it will take you far longer to divert via ABZ than to take the replacement bus to Perth. And if you wait to buy your tickets and route through ABZ that would cost more now, it could cost a shed load more then.
So if the schedule changes it will still be far faster and cheaper to stick with their route than to devise your own.

You've been itching for the advance fares- grab them now and ignore the hypothetical may be's.

This is overthinking.

Haha, yes, that's me, paralysis by analysis. I did buy my train ticket LEU-INV since I saw no further delays. I will go ahead and buy my INV-EDI. Thank you for all your knowledge and help.