My cellphone has a built-in GPS receiver. I thought it might be fun to use it to find out the speed & bearing of the trains I'm on. However I can't get a signal inside any train (C3/M1/M3/M7), despite holding the phone up to the window. The phone's instruction manual states that certain glass coatings will block the GPS signal. Are the train windows coated or treated in some fashion? Has anyone else had any luck using a GPS receiver inside the train?
The frequencies that GPS uses are very easily sheilded by any material, especially the body of the cars, and without a supplemental antenna (looks like a mushroom about 3") you wouldn't get enough of a signal to get a fix from several birds, unless you were lucky and had several near the horizon to penetrate into the window. The coating on the windows shouldn't attenuate the signal too much, but as mentioned earlier it would have to near the horizon.
The M7's utilizes GPS to make the automated station announcements, but even that would have to be set manually by the crew if underground or unable to get a fix for whatever reason...