Train from Monterosso to Pisa

That is likely true

You can look at the previous Sunday Oct 11 and see what train schedule to expect on Oct 18
They don’t change much if at all

These are all mostly regional trains so no need to purchase in advance-price never changes and they can’t sell out

Trenitalia is showing a big detour between La Spezia and Pisa on Saturday and Sunday, which leads me to guess that there is a weekend track project on the line. Normaly it is less than 2 hours between Monterosso and Pisa, but it is showing several connections and taking 4 to 6 hours Saturday and Sunday with trains routed into the mountains rather than hugging the coast on the main rail line.

Where exactily do you find updates on train track upgrades or repairs. I wonder if this is the problem I am encounterining. If this is the problem....then it is helpful to know today rather the day of. Many thanks.

I also suspect there's some kind of maintenance scheduled on that weekend, as the trip to Pisa is usually considerably shorter. I checked schedules as early as 04:00 and all the rail trips are ~4 - 6 hours.

You might check the FlixBus Italy website as they appear to have trips from La Spezia to Pisa on Sunday, October 18th. The fastest trip departs at 10:55 and takes one hour. You should be able to get an early train from Monterosso to La Spezia in time to make that connection. I didn't check on their pickup or drop off locations in each city.

Forget AI and use common sense. Make two searches on trenitalia.com putting Sunday 11 and Sunday 25 as days of travel.

If the system shows less runs and "strange" routes only on the 18th, it means the search engine is trying to make you bypass some track works on that specific Sunday. Only some work-stages near the tracks force Trenitalia not to use a line, I'd wait a couple of weeks and see if they can complete these works earlier than expected and let more trains run.

The planned works on the RFI network are listed here by region. Language aside, the line you are taking can be affected by works happening in 5 different regions.

Often when a line is closed for track works there is a parallel replacement bus service. Entering these busses in the timetable is however not something that Trenitalia is in a habit of doing promptly. So it is well possible that they will only appear on a week or so...

I took the train from Manarola -> La Spezia and then La Spezia -> Pisa last week on Saturday. I bought my tickets the day before, and there were plenty of options for trains throughout the day. I want to say 6 or so. Ours was a noon train from La Spezia that got into Pisa a bit after 1:30.

Ah, but no track work projects were taking place when you traveled.