In most countries with 250km/h (155mph) plus high speed rail, it's taking back marketshare from the airlines --- because it tends to be quicker, by virtue of not having to arrive at an airport an hour before departure, not having the hassle of security, etc.
To use the typical North Eastern corridor example, it'd be far from impossible to build a line from New York to Washington DC, with one stop in Philadelphia, that would run in under two hours. This is half the time of driving between the two, and only half an hour slower than a plane (and quicker than that once you consider airport security, etc.).
A broad statement like cars and planes making trains irrelevant is untrue: certainly I wouldn't want to do a transcontinental journey by train, but there's no reason why trains can't better compete with air, instead of just competing (time-wise) with cars.