It's taken 40 years, but Boxcar Willie - a tall, tough, Texas teddy bear of a man - has made it in the country music business. A platinum-record holder with his ''King of the Road'' album, Boxcar is the 60th member of the Grand Ole Opry and a champion concert-giver - 270 appearances in the last year.

Everywhere he goes is an appearance, it seems. Here Boxcar spent two days traveling in a limousine to meet Washington VIPs (''the vice-president spent 30 minutes with me!''). He wore his hobo gear - blue striped overalls, pink shirt, red bandana, and scruffy shoes. Then there's his alpine hat, covered with buttons and mementos that he claims ''used to weigh 10 pounds at least; I cleared off a whole bunch before I came up here.''

He was wearing this outfit when he and his entourage - his manager, a Smithsonian guide, and his wife, Miz Box (he calls her ''Mom'') - pulled the limo up to McDonald's for lunch. There he signed autographs (''I never get to eat in peace''), munched a hamburger, and gave his napkin to a bag lady who was using the papers to cover her hat against a seasonal chill in the air.

There seem to be two kinds of people in this man's life: total strangers, who are called ''sir'' and ''ma'am'' in a drawl baked in during his childhood in Sterrett, Texas; and people he has met, who are called by first names only. If you're female and he's met you before, you get a hug. If you're a child, you get a present.

In this, Boxcar Willie emulates one of his idols, country music star Roy Acuff. ''He's a human being - he considers the feelings of others,'' he said of Mr. Acuff over lunch before a Smithsonian concert. ''People don't come too small or too big for Roy Acuff.''

He also admires Acuff's willingness to ''stick to his guns'' and remain a country music singer in a business that encourages people to switch to rock-and-roll - and, lately, rock-and-roll singers to switch to country.

It's that last tendency that blew Boxcar out of 40 years of ''holding other jobs and playing music on the weekends'' into full-time music. As a flight engineer in the Air Force and a disc jockey in Fort Worth named Marty Martin, Boxcar would ''work honky-tonks for nothing or next to nothing, just trying to get ahead.'' He started at 10, playing hooky from Boy Scout meetings to sing with his ''rowdy'' Cousin Joe in Dallas honky-tonks, and he used to work with other unknowns - like Willie Nelson, who once ''played the guitar for me,'' he says.

That association has led to the rumor that the ''Willie'' in Boxcar's name came from his fellow 40-year-overnight-success singer. In fact, Boxcar Willie - who was born Lecil Travis Martin, named for ''an old hobo friend of my daddy's'' - grew out of a dream he had in the early '60s. He grew up in a house built ''not six feet from the railroad track,'' and always admired the hobos who rode the rails. ''They'd knock on back doors and offer to chop wood or work around the house for a meal. They were trustworthy, honest - they'd rather die than steal,'' he has said elsewhere of this breed.

The hobo costume was made up in 1968, but it languished in a box under the bed until the 1975 Country Music Association Awards show, when something snapped. ''This is a table, and this is a chair,'' he explains, knocking on restaurant wood. ''You can call this a table if you want to,'' he says, pointing to the chair, ''but that don't make it a table. Same with Olivia Newton-John, George Burns, all of them. We love 'em, but they're not country-music singers.''

After watching the awards go out to people he doesn't call ''country,'' he announced to his wife that he would ''win a major country music award within five years.'' Then he donned the dowdy hobo getup and walked out as Boxcar Willie.

One of the increasingly rare few who make it in the music business without the backing of a major record company (''King of the Road'' originally sold in the United States by TV ads), he describes the ascent in metaphor: ''It was like trying to get an education without knowing how to read. Ever try to carry a big armload of socks without dropping any? It was frustrating.''

Working clubs in the New Mexico-Texas-Oklahoma territory, Boxcar would occasionally foray out to Los Angeles. There he stayed with his cousin, actor Tommy Lee Jones, while doing the ''Gong Show'' (''it was a way to get national publicity'').

Then he got his big break in one of those quirky ways: He was asked by George Jones, a man he describes as ''one of the giants of the business,'' to sing at Possum Holler, Mr. Jones's club in Nashville. When Boxcar arrived, he was told the club was booked up and it was doubtful if they could squeeze him in.

In fact, many of the acts were ''having personal differences,'' as he has phrased it, and Boxcar stepped on stage to fill the bill. In the audience was Drew Taylor, a Scottish promoter booking a tour of the British Isles. He hired Boxcar and later headlined him when the main act canceled out.

The British ate him up. His 1979 album, ''Daddy Was a Railroad Man,'' was voted British album of the year, and ''King of the Road,'' released there first, sold over 100,000 copies.

It's not his singing, exactly, that brings Boxcar his distinction, but an odd noise he makes deep in his throat which sounds exactly like the tooting of a train. Ask him how he does it, and he smiles. Ask his fiddle player, and he tells you, ''It takes a lot of gargling.''

We are talking here about a man who makes his living by sounding like a train. It's a noise he works in often and well in his concerts, singing songs like ''Wreck of the Old 97,'' ''Hand Me Down My Walking Cane,'' and one of his favorites, ''All Around the Water Tank.'' Of the latter, he says, ''it has everything - heartache, poverty, corruption, blackmail, geography. . . .''

Between tours with his four-man band, the Texas Trainmen, and his traveling Country Music Museum, Boxcar relaxes at his Texas ranch, where he's ''just Daddy'' to his three children. But soon he's back on the road again, making some of the 200-plus appearances scheduled for 1984.

How long will Boxcar Willie travel? ''I'm gonna keep it up until they don't want me anymore,'' says the 40-year overnight success.