The only video recorders at the time were huge Ampex reel-to-reel machines used by television stations to rebroadcast live shows in later time zones.

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Some TV executives and analysts suggest that online video, tablet computers and wider use of digital-video recorders could be pulling prime-time viewers to other places and times that aren't counted in prime-time usage figures, although many also caution it is too early in the TV season to draw any broad conclusions.

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Mr Heilemann (who once worked for The Economist and is now at New York) and Mr Halperin (of Time) have been diligent with their tape-recorders.

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Now one of the major manufacturers of data recorders says the company will look to extend their battery life to give authorities more time to find them after a crash.

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Data recorders on board are expected to provide the speed of the Metro-North trains at the time of the crash and other information, he said.

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For the first time ever, the industry will take into account viewers who watch shows after their original airing on digital video recorders (homes with these devices make up some 20% of Nielsen's sample).

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