TSA sets the pattern. Already Homeland Security has been making grants to regional bus lines based on the condition that they adopt TSA-approved security rules. TSA can't itself cover all buses, trains etc., but soon the time will come when TSA-approved security regimes will be implemented by local governments as a condition of federal Homeland Security funds.

Even that would not give the resources to search all travellers but it would insert random government harassment as a possible factor into all travel other than by private vehicle, but as noted above, there already exists sufficient technology to track every vehicle in the nation 24/7/365.

TSA could readily make a Chertoff-style "grant" to On Star, on the requirement that data be shared with TSA. Combine that with a requirement that only On-Star (or similarly tracked) vehicles be allowed to enter the airport perimeter, perhaps with registration on line linked to boarding passes.

Since there has been at least one vehicle attack at an airport (Glasgow) and maybe others that I can't think of right now, the legal and political justification for this already exists, at least if one accepts the idea that fear should run our lives.