Perche

No, you might be able to get on and off quickly for a macchiato or a glass of wine, but you will either have a four hour time limit on your ticket. If the next train will get you to your final destination within four hours of departure, that is OK. If not, then you will be ticketless. If you are on a train with assigned seats, it will say carroza, or car number, seat number and time. If the conductor comes and checks your ticket and you are not on that particular train, you are considered to be without a valid ticket. If on a train with an assigned seat on a specific date and time, you must stay on that train.

You mentioned that your final destination is Cinque Terre, but you are going to La Spezia. Monterosso is one of the five towns of the Cinque Terre. Do you really want to go past the Cinque Terre to La Spezia, then turn around and go back to the Cinque Terre?

Once you are in Monterosso you are in Cinque Terre. There is a local train that connects the five towns, and that actually ends in its last stop outside the Cinque Terre in La Spezia. Whatever your final destination is in Cinque Terre, you can get off in Monterosso, go for a swim, do what you want to do on their excellent beach, then take the local train for a few euros to whichever one of the other four Cinque Terre towns you are staying at (I assume you are not staying at Monterosso, because you said you just want to stop there); Riomaggiore, Manorola, Corniglia, or Vernazza.

If you are specifically going to La Spezia then your destination is not Cinque Terre because it is not one of the five towns. You might be day tripping to CT, which is OK if it is not in high season. But no, sometimes people take a shot at pleading ignorance if they are on the wrong train, but you usually won't get away with it. You are going to have an assigned train even if you don't have an assigned seat on base fare, or you will have an assigned seat and train. You can't get on and off. If you really want to get off on Monterosso you can buy a ticket that takes you there, then take the local train to La Spezia for a few euros.

ckendall

I would assume so; if you get on the Milan-La Spezia train that you have the ticket for at Pavia and get off at Monterosso I would imagine there would be no problem. When the conductor first passes through after Pavia you may need to explain that you got on late but I would imagine that would work (and I have never seen anyone check where you get off if you get off before the final destination of your ticket).

Perche

You can definitely buy a ticket to anywhere, and get off wherever you want before your final destination. But from Pavia to Monterosso the only way to go by train is to buy a ticket that will be on a specific date and time, in an assigned car, in an assigned row, in a specific seat. You can get off wherever you want. But you can't just get on the next train, even on the same date, even if the final destination is the same.

If OP buys a ticket on the 12 PM train A that leaves Pavia with a seat assigned in car ten, row 5, seat B, and a conductor finds him on the 3 PM train B, in car 5, row 2, seat C, he will be considered as ticketless and fined. You have to stay in the train that you bought the ticket for. Conductors really don't care if you change seats, but you have to be on that train.

On Flyertalk there is the occasional thread about someone buying seat in Y and trying to sneak into F. That doesn't work out. One of the few aggravations about italian train travel is buying a premium seat, having a row to yourself, then having a family of three come and occupy your row coming up from base fare. Conductors usually tell them to go back, but they are often ignored.

However, if a conductor finds that you are on a different train than the one you bought a ticket for, you aren't going to get away with that.

In other words, if the OP wants to buy a ticket from Pavia to La Spezia, and wants to get off in Monterosso, no one is going to stop him. But if two hours later he decides to get on the next train from Monterosso to La Spezia, and he hasn't bought another ticket for that specific train, then he is traveling without a ticket.

In other words, going from Milan to La Spezia, and getting off at Pavia and Monterosso, will require three separate tickets. Of this I am sure. You might get away with it if the conductor doesn't find you or looks the other way (rare), but it's like jumping a turnstile in a NYC subway. You're not supposed to be on a train without a ticket for that specific train unless you are on a local, where you can travel on any train you want for a period up to four hours from the time you validate your ticket.

YVR Cockroach

No, you still don't get it. I am talking about a specific train X - which is scheduled to leave city A at GGh and arrive in City D at JJh- on a specific day, day Y. Train X being IC ABC or EC BFG or whatever specifically-numbered train. Getting on (once) at B and getting off (once) at C between the two ticketed cities, A and D. Not talking about taking any different trains but just train X only on Day Y. Not necessarily between A and D but getting on an intermediate station B and getting off at intermediate station C.

Never said anything about taking trains I wasn't ticketed on.

Perche

OK, go ahead and do it, let us know how it works out.

If you buy a ticket for a train to go from point A to point C, and on the way that train stops at point B, you cannot get off at point B, and then continue on to point C on a later train.

On the way to point C the train may stop at point B for 5 minutes, just long enough to let people get on and off with their luggage. If you get off for 6 minutes to go get a cafe and that train has pulled out, you need to throw the ticket you have into the recycling bin, go to the ticket office, and buy a new ticket.

You cannot get off the train except to go out onto the platform and stretch for a minute or two, providing you keep a sharp eye on the conductor to make sure he's not about to close the door on you which will make you have to buy another ticket.

Punto finale.