You're approaching the railroad tracks when you hear the horn and see a train coming down the line. Most people get annoyed by the delay. But if you relax and look forward to watching the train roll by, you're a railroad buff.


The condition is probably hereditary. Other writers wax rhapsodic about playing catch with Dad in the back yard, but I was a youngster in the last days of steam locomotives during the 1950s.


When the family was out on a Sunday afternoon drive, wandering along the back roads and farm towns of Weld County in the fall, we might see coal smoke on the horizon, or hear the low moan of a distant steam whistle. Then my dad would race toward those tracks.


If he got there in time, we might see a Burlington steamer brought out of storage to handle the rush of harvest business. Or the Great Western Railroad's immense and immaculate No. 90, straining to pull a long string of sugar-beet gondolas to the mill.


Often the county road ran parallel to the tracks. We'd pace the train, waving to the engineer, and he'd give us our own toot from the whistle.


Those are among my earliest memories, and though many of the memories concern railroads, not all are of steam locomotives.


Once I was walking home from a Cub Scout meeting after school. At the railroad crossing, an idling sedan was parked. I heard the whistle of a crack UP express en route from Cheyenne to Denver, and decided to wait and watch the train go by.


Over by the sedan, there was a strange little tower I hadn't noticed before. It looked like a steel question mark, 10 or 12 feet high. Suspended inside the curve was a leather bag.


As the train roared by, I heard a loud "thwack" and saw a metal arm reach out and grab the bag from its perch. Out of the train flew another bag, which landed on the ground nearby with a big thud. The postmaster of Evans, Colo., got out of his car and picked up the bag.


I was curious, but he got to me first. "Don't ever stand so close when the train comes by," he cautioned. "That thing's going about 70 miles per hour when they toss out our mail bag, and if that bag ever hit you, you'd be one dead kid."


After I assured him that I would never again stand that close to the tracks (a promise I have generally kept), he explained how the postal clerks worked right on the train, sorting letters inside the mail car.


At big cities like Greeley and Denver where the train stopped, they loaded and unloaded mail at the station. But tiny Evans didn't even boast a depot, let alone a scheduled stop. The postmaster put the outbound mail on the hook for the train to grab on the fly, and picked up the bag of inbound mail the train tossed off.


It was luridly fascinating, just like the depot in Greeley, where a telegrapher once let me use his key. Or the yards in nearby LaSalle, where occasionally we kids would get cab rides from the switch engine operated well into the 1970s, and I got to drive it on a winter afternoon - working the throttle, tooting the whistle, shoveling coal. Or riding in the caboose on a Great Western mixed train (85 cents for a round trip to Eaton), sitting in the cupola and delighting in the passing corn fields.


Or just standing by the tracks when a passenger train roared by, waving at the passengers and wondering who they were and where they were going, and wondering whether they wondered about us.


There are people who buy (and carry) radio scanners tuned to the channels that railroads use, so they can hear the engine crews talk. Others study the distinctions between types of diesel locomotives. As with all such American compulsions, there's an industry to cater to it with trade fairs, swap meets, videos, books, timetables both original and reprinted - if it's got a railroad connection, somebody has it for sale and somebody wants to buy it.


*E. Q.