Train To The Plane

At Bradley (BDL) there is the Bradley Flyer quasi-express bus at a $1.75 local fare running from Hartford Union Station ($15.75 for 10-trip, $63 monthly).

BDL is in the town of Windsor Locks, and is actually closer to the station there and a shuttle from the Windsor Locks station to BDL would only be a 10 minute ride. An opportunity with the station relocation coming soon.

At Rome, Italy, Leonardo da Vinci Airport has direct service to TERMINI station in downtown.
In Frankfurt am Main, Germany, the Flughafen has direct service to the Hauptbahnhof. From there, you can get to U or S Bahn tracks to get anywhere in metro Frankfurt, or an E, D, or ICE to anywhere in Germany.

Colonel, add Munich (MUC/EDDM) to the list of "train to plane".

The S-Bahn, which can be ticketed as part of a "mainline" journey is X a Plaza from Terminal 2 (United, Lufthansa) and can take you either to the Hbf or to Munich-Ost.

While I'm not "going over" this year (83yo), I found such to be quite convenient (in Salzburg, a 900m walk to the hotel).

Maybe I'll "feel up to it again" but I'm not all that optimistic.