There are plenty of advantages to having train tickets sold through a common booking platform with an airline.
True, a passenger can book both tickets independently, but you’re adding a step which a lot of people aren’t going to necessarily want to do. For those who will, you are relying on them keeping track of scheduled arrival/departure times to make sure they book the right connection, rather than having it already built in for them. Making people go to two different booking platforms to book a trip is going to reduce the likelihood that they are going to bother with such.
This likelihood is also heavily impacted by whether they are even aware of the train option. Yes, in theory you could heavily advertise the connection in the in-flight magazine. However, most people don’t read the in-flight magazine.
You could heavily advertise in the connecting airport, but at that point you’ve already missed out on getting them for that trip (and you have to hope they think about it for the next trip, again putting the onus on them to book through two different platforms).
The other, bigger issue, IMO, is that you are also going to miss a ton of potential passengers who could use the service as a connection but never use the connecting airport where the train connection exists.
Take a hypothetical route for which someone is traveling to Philadelphia, and suppose there is a nonstop to EWR from their origin, but no nonstop to PHL. They go to the airline website and search XXX-PHL, see no nonstops, but see the best connection as being through ORD. They will never see all of the Amtrak ads at EWR because they never set foot in EWR, and would have never thought to book to EWR and then check Amtrak to PHL. Instead, they’re connecting through ORD. And, practically, there is no reason for Amtrak to plaster ORD with ads about NEC connections via EWR, because from ORD you can fly nonstop to each of the relevant cities anyway.
The problem with Amtrak’s EWR service is that it’s an “if you know, you know” proposition. Granted, lots of people ride the NEC and, thus, would know. But lots of folks from elsewhere don’t know the first thing about Amtrak and would never for a moment think of booking their east coast trip to EWR if they’re not traveling to New Jersey or NYC.
If they want to travel to New Haven and see that their best option, according to the airline, is to connect in Chicago and wind up in Hartford, they will never even realize that they have a nonstop flight plus train option.
Even international travelers who are used to taking trains around their own countries won’t know that EWR (or, I suppose, BWI) are air-rail connecting points for travel up and down the NEC. So they might book a connecting flight rather than a nonstop to the train connection.
I think the biggest limitation in this is probably booking systems integration and/or working out the financial aspect of revenue sharing for these tickets and cost sharing for reservation system maintenance. Otherwise, there’s no really good reason not to join up.
And if *Gold gets me into the Metropolitan Lounge, I suppose that’s fine with me, too.