IIRC the collision resistance is a special requirement for for American trains, as other countries put the effort in upgrading their signal systems so that trains almost never collide in the first place.
No, "collision resistance", by which you mean longitudinal resistance, is a requirement of every train anywhere, because it's not only about collisions. US regulations are just higher than other countries. But all trains are stronger in this particular way than airplanes are. Also, trains have requirements for specific things like skin buckling resistance and sudden external pressure changes that I doubt any current airliner could meet either. (The sudden pressure change when going through a tunnel or passing a foot from another train I'll bet would shatter the outer passenger window panes on basically any airplane. This force is equivalent to swinging a sledgehammer against a window at high speeds.)
Airliners are made to be as light as possible given the aerodynamic forces they have to deal with. Trains are just build to withstand different forces, and while weight matters, it doesn't matter quite as much so most trains are heavier than they probably really need to be (because of things like window size, seat size and comfort, flooring material and interior fittings, etc.) So while you could probably make an airplane that converts to a train, in the same way there are trucks that turn into trains (multimodal is not a new idea), you'd end up with something that's not a very good plane *or* train. It would be a very heavy airplane and a very slow and not very comfortable train. You'd probably have to have some pretty severe speed limits on it.
I think that making it work as a train, where all you'd need would be for it to be just good enough to go those last few miles into city centers, would be less of a problem than making it a workable airplane. The economics of that just wouldn't work if you have to build it to withstand the forces of both an airplane and a train. There's no way it'd make any money.
I'm tired of being a wanna-be league bowler. I wanna be a league bowler!