Yes, the 1955 tests, at up to 331 km/h, were dramatic, but "the track was seriously damaged after the high speed runs," according to La Vie du Rail and quoted on www.railfaneurope.net. As a result of this test and later TGV tests, a stretch of the TGV Atlantique line was built with especially large radii, deeper ballast, changed rail cant, etc., and higher tension in the catenary wires for the 1990 high-speed tests. As you point out, the infrastructure did indeed not suffer much. Some track realignment was necessary after the tests, and the wires had to be adjusted, if not replaced. The tests showed that these high speeds are feasible and safe, but the infrastructure would have to be built at a much higher cost, and much more powerful trains would have to be built; now the thinking is that 350 km/h is the highest practical speed for steel wheels on steel rail at a reasonable price. The 1990 test train was also much shortened from normal sets, and the suspension, wheel size, etc., etc. were changed. No standard train set today can run at 500 km/h.