She was not preparing for a tae kwon do bout, but performing her personal version of the underground battle engaged in daily by millions of New Yorkers: reading, intently, on a sardine-can D train heading swiftly toward Brooklyn in the evening rush. Without holding on. "I am a New Yorker," the woman, Robin Kornhaber, 54, told me as if those five crisp words explained everything. "I can do anything on the subway.