Electronics/Installation of Intercoms in long distance express trains.
Is it technically feasible and useful to install Intercoms in Long distance express trains?
Yes. For example their are communication systems on the ShinKanSen bullet trains in Japan. They are more of annunciation that open two way intercom. They allow officials to make announcements and promptings in all cars and for passengers having trouble to communicate with train officers.
I think other train service in Western Europe do also have such functionality.
It's use is obvious from a standpoint of rail company passenger comfort and explanation of route, stopping points and when station is approaching.
But for passenger to passenger it is not such a need on either long distance or short distance, I think.
What were you thinking. Were you worried about technology being installed in train cars? It would be not problem; there are many kinds of wireless and wired systems that could provide a solution.