We live within the sound of trains. We always have, through five different houses and five different states, West Coast, Midwest, and East Coast. And the trains sound the same whether we are listening in Maine or the Main Line, Illinois or California, for their sound seems to transcend locale.

The percussion of railways always leads me back to my childhood summers by the Saco River, listening to the nightly freight train running from Portland, Maine, to Berlin, N.H., winding its way up the river valley through Crawford's Notch. Its forlorn wail echoed down to our cottage, and as I lay in bed I could imagine it snaking along the trestle beneath the Frankenstein Cliffs, just below the grand mountain notch.

I love being stopped at a railroad crossing. A train tells you where it is coming from. Freight cars evoke distant landscapes and diverse industries. Airplanes are about destinations and swift arrival, whereas trains are about the straight line and the sinuous route, the rolling journey along the traces of past commerce. Trains have the romance of names from story and song: the Flying Scotsman, the Orient Express, the Rock Island Line, and Lionel.

That last one was the name on my childhood model train. And those memories prompted me, many Christmases ago, to revive the train tradition with my own children, when a magazine ad for model trains proved irresistible. "Lehmann-Gross-Bahn - The Big Train," it heralded. For an enticingly fair sum, we too could have the large, G-scale model railway from Germany circling beneath our tree - for the pleasure of the kids, of course. I ordered over the phone, had it delivered to my work, and then set up the track on my office floor. Ah, the sound of trains, even model electric trains. Wheels on rails transport you back in time, and away.

The train store included an encyclopedic catalog of European and American model railroad lines, magnificent garden layouts with elegant descriptions in German. I had purchased, evidently, more than a cute train for Christmas. This would be a life work, and its sounds would become a mnemonic and ritual for Christmas, childhood, parenthood, and family journeys.

From that year on, two weeks before Christmas, we went to the train store to buy another railroad car, perhaps a few sections of track. I let the kids choose the additional "rolling stock," as we aficionados call it, and the store had a myriad of choices: cattle cars, boxcars, flatcars, functioning tank cars, and crane cars, to say nothing of the diverse passenger lines. (We haul freight, not passengers.) And every car announced a new location - ours - as we moved around the country.

Year one, the layout encompassed the living room. Year two, we expanded into the dining room. Eventually, I hoped, the whole downstairs would be a maze of tunnels, switches, homemade villages, and logging operations. At this rate of expansion, I thought, transcontinental operations will occur by the time our youngest child goes to college. And I'll finally get to play with the trains by myself. For the time being, I was happy if the dogs weren't lying across the track or Ariel, that youngest kid, wasn't blocking tunnels with her baby buggy. Ariel Rose just loved a good train wreck.

We added a red caboose; then the red- white-and-blue State of Maine Potatoes boxcar. Rock Island Line, Frisco Line followed. Our model railroad followed us around the country. And we would always hurry home to hitch up the new car and get it rolling behind the little, yellow, muscle-bound locomotive. Clickety-clack - a new sound in the chorus of Christmas train travel.

When the kids were in bed, I liked nothing better than to turn on the Christmas tree lights and lie by the tracks. This is Christmas: the tree, white lights, cold weather, and our Big Train clacking along in circles, delivering lumber and graham crackers to towns up and down Nelson County. The aficionados would frown. None of my homemade tunnels, depots, and houses conformed to scale, or anything remotely reminiscent of our train's actual historical milieu. We haven't, in fact, run a true railroad at our house.

Now, the MIT Model Railway Club runs a railroad. One year, I took the kids for a visit. After walking hundreds of yards through a maze of physics labs and arcane clubs housed in an old warehouse, we found the apotheosis of model railroaders: half a dozen men in engineer caps, some with hair in ponytails, mastering a railroad universe in a seedy top-floor room with clanking radiators, trestles, and a Hewlett-Packard mainframe computer blinking in the corner, confounding train wrecks. Their advanced degrees and patents were assumed.

Here were ancient and modern freight trains, passenger trains, trolleys, an el; a switching yard, a turntable, mountain tunnels. The room hummed and clattered with the sound of trains. Even such a small-scale model train could sound like that Berlin-bound freight approaching the Frankenstein Cliffs, or stock my kids' imagination for the future. You could journey from urban to rural landscape and back, and to and fro in time; from multigenerational childhoods to young adulthood to my present reminiscence.

I'm going to buy a boxcar this year - yellow, with "Pennsylvania" on the side, the missing state in our family journey. There are no kids at home now to help set it up, but that's OK. I'll take my usual place under the tree and ship myself back and away, to all the Christmases past and all the towns with trains. The Christmas express must keep on rolling, freighted now with cherished memories.