A letter from Northwest tells it all and tells it well. Here's the full text, except for the author's name, which she has asked me to withhold:

"Dear Bob:

"May I follow in the inky footsteps of others who address unknowns through your column? I've a word for a youngish couple seen in the Metro Center station at its 13th Street entrance on a recent Saturday afternoon. If I hadn't worn my ugly clodhopper shoes that afternoon, your beautiful baby might now be maimed or dead.

"They seemed to be trying to figure out the Farecard machine when we saw them. The baby was crawling away (across the filthy subway floor) as fast as he could and the mother stopped her study of the machine just long enough to send after him a toddler who could not have been more than two. By the time we realized that the little girl was not stopping her brother, he was maybe four feet from the up escalator.

"I never ran so fast, though it still seemed to take an eternity. His little fingers were about to touch the bottom step when I snatched him back. They could have been caught and dragged him -- well, I hate to think of it.

"Did the parents even see this? You'd think the sight and sound of a 74-year-old woman suddenly racing through that almost empty space would attract attention. But another woman came up to me -- at first I thought her the mother -- and took the escalator before the real mother approached.

"She was well-dressed and well-spoken (received the baby with a cool, 'Thank you very much'), and seemed unimpressed when I said, 'He was almost on the escalator.'

"Perhaps they were tourists. Perhaps they don't know a child was killed on a Metro escalator not long ago. Perhaps they don't think of kidnapers, either. But I hope they've remained in town long enough to read this and learn by what a thin thread fate took over their responsibility.