The economics of high-speed rail are rather interesting. The cost is capital cost of upgrading the tracks. When it comes to higher cost of operating faster, there isn't any -- it's actually the reverse: faster running improves the economics.
This is because you need less trainsets for the same service, and less labor costs. If it only takes an hour and a half, Boston - Portland, that's an hour less labor from the crew, and another few trips that can be squeezed from the same equipment. Add to the mix that the faster trip means the train is more competitive and can therefore charge much higher fares. Most high-speed routes actually make money and some in fact provide a reasonable financial return on investment.
There's certainly an environmental cost to running faster, in terms of increased fuel use and therefore pollution. It's rather striking. If you are going to go hard-core green, then we shouldn't fly at all and shouldn't have fast trains and shouldn't drive over 55mph. America (and myself) are not at this point, though.
If you want trains to compete against the auto and planes, speed helps considerably. Vermont studied the Boston - Montreal route (http://www.aot.state.vt.us/Planning/Documents/Planning/Final%20Report%204-30-03%20Boston-Montreal%20Chapter3.pdf) and found that at 59mph the route would see 213,276 riders a year, but at 110 mph (and speed restrictions for curves, etc) would see 683,667.
