Originally, it was a football fight song -- written during a ride on a Japanese commuter train 32 years ago.

"I didn't know a note of music," says James Duff, 68, writer of Army training manuals at Fort Belvoir and a former football coach who still doesn't know a B-flat from a C-sharp. What he does know is a good song when he writes one.

Duff, who was with the U.S. Occupation Forces in Japan when he first penned the fight song, had enough confidence in the musical number to sing it, over more than two decades, to anyone who wanted to listen and more than a few who couldn't avoid it.Since 1974, however, Duff has almost had to be asked to belt out his gridiron chorus: "Just watch our line come charging down the field. "Just watch our backs go crashing by."

It's not that Duff has suddenly grown shy. It's just that his new song, a paean to the U.S. Army Corps of Engineers set to the same music as his football song, now holds center stage. And this one, Duff hopes, will be sung for the ages.

Duff, a civilian employe at Fort Belvoir, is trying to persuade the top brass there to replace the corps' official march, "Here Come the Engineers!," with his own "Thumbs Up! We're the Fightin' Engineers!" The old song, written about 40 years ago, is just not very peppy, says Duff, whose own march is spirited enough to make John Phillip Sousa shout.

"You can sing along with me if you like," says Duff, an Ed Wynn look-alike, as he switches on a cassette recording of the music to accompany his steady voice. "We bridge the rivers swift and wide. "We clear the way for tanks and infantry. "All obstacles we blast aside. "We build the best damned roads to hell and back. "Our thumbs are up, by golly. "Yup, we're the fightin' engineers."

Duff is one of Belvoir's genuine personalities. In a uniformly khaki world, he is an original. An unabashed storyteller and singer of songs, he is the official crooner of "Happy Birthday" at the monthly coffee-and-cake parties in the Training Literature Division. For 21 years, he was a member of the Corps of Engineers, and he stayed with the Corps -- as a civilian employe -- after his retirement in 1966.

The division's 50 employes translate technical jargon into training manuals Belvoir's soldiers can understand. Wording instructions on how to operate a bulldozer for soldiers of 7th-grade reading level does not demand peerless prose, admits Duff. So he has his music to transport him.

"It's time to let people know about Jim Duff and his song," says Fred Goldberg, civilian information officer at Belvoir and one of Duff's good friends. Goldberg worked with Duff in the Training Division until last year, when he was appointed to the present office. Now he is doing his unofficial best to see that "Thumbs Up" is officially adopted.

A press release Goldberg issued last week already has resulted in one newspaper interview for Duff, and a few nibbles from local television stations. Duff has taken all the attention with characteristic good humor.

"I'm holding out for Merv Griffin," he says.