Risks to Not Using Second "Leg" of Flight Ticket that Includes a Train??

I don't see how they can prove whether you took the train to Stuttgart or not. It is not as if they are demanding to see your cancelled train ticket before allowing you to check in for the return flight. Even if they did (Italians asking to see a German train ticket?), the ticket inspector may not have got to your part of the train before it reached Stuttgart.

P.S. Customs is not an issue, takes no time and is just a "walk through". It is "immigration" (passport control) which take the time, and where they stamp your passport.

Thanks -- changed "Customs" to "Immigration" in the original post.

The Lufthansa customer service rep basically told me the same thing over the phone until she asked her supervisor, who not surprisingly said the whole ticket needs to be used.

That is interesting since it involves a train. If it was strictly the second leg of an airplane flight you would have a problem because as soon as you are a no-show the rest of your ticket is canceled. However, I am sure they would know if you used the train tickets since the conductor scans the ticket into his wireless, handheld device and it is so easy for computers to talk to each other. During the past few years our tickets have always been scanned on the German trains include one bus operated by Bahn.

So you are gambling that you don't see a conductor or that the ticketing systems are not connected. I don't think anyone can advise you one way or another. I think you have to decide if you are willing to gamble the $600 saving against the full price of a return ticket should they cancel your return for not using the train. It is your dice roll.

They do scan the rail tickets so I assume they would know you didn't make the train. Why not just fly into Frankfurt and train to Stuttgart and book the car in Stuttgart?

The reason I would not take the train to Stuttgart is that it would unnecessarily add ~1.5 hours waiting for the train and 1.5 hours sitting on the train when I could have been on my way from Frankfurt.

But if they actually scan the train tickets, it's not a risk I would take. I've been on plenty of trains in Germany where the conductor never makes it through the whole train, so canceling the return trip seems like a very harsh penalty for something that is outside of the traveler's control. But, probably not worth risking it in our case.

Navigator, you are not gambling that "you don't see a conductor", since you would not be on the rain. You are gambling that they can't definitively say the conductor checked every single passenger on the train before the train got to Stuttgart. They cannot prove a negative (that you were NOT on the train).
The object of the conductor scanning your ticket is to prove it is valid and has not already been used. I Very much doubt that the DB system reports back to Lufthansa and this information is available at check-in at Venice.

Check out Delta flights from Atlanta directly to Stuttgart. I took that flight in 2002. At that time it was the only flight from NA to Stuttgart. I think they still have it.

If it is anything like the Air France plane/train combo tickets, you do check in for the train at a special airline counter in the airport train station, airline personnel are there to check you in, they transfer your baggage as they would for any flight, you wait in a nice, separate waiting room--ie, it's handled just like a flight, supervised by the airline.

If Lutthansa handles this the way Air France does, then missing the train would make you a no-show, cancelling all the rest of your ticket.

As some others have said, if the train ticket is part of a ticket issued by the airline, I don't think it's a good idea to risk it. The train station at FRA is inside the airport, so you won't have to leave the airport to take the train. If your plan doesn't work, you probably will be spending a lot more than $600 for a full-fare return ticket (should that be your only option).

Thanks everyone for the thoughts and advice. I want the trip to get off to a good start and decided not to take the risk and take a different one-stop flight into Stuttgart for about the same price. So, in the end I'm really just losing a few hours of time and actually was able to secure a better, shorter return flight on the way home, so it all works out.

I didn't post earlier on this, and it looks like it is resolved, but for future reference. If that were a normal Bahn ticket, received by mail, the conductor is not going to know the situation and he would just stamp the ticket like any other ticket. However, if it were an online ticket, then it would probably have a bar code, and he would scan that (I have had online tickets scanned), and it would be entered into the computer, so they would know. The ICE from FRA to Stuttgart takes over an hour. I'm sure that is enough time for him to check all of the tickets.

Whether you fly from FRA or take the train, you will go through immigration at FRA, because at that point you are entering Schengen.