The 1.7-mile Howard Street Tunnel that billowed smoke yesterday is not a prominent part of the Baltimore landscape, not a source of great civic pride. Yet the tunnel, mostly ignored and unseen - even unknown to many residents - is hugely important, the way rail freight basically gets from here to there along the East Coast.

And it's hardly just a functional workhorse. The 106-year-old tunnel is distinctive in many ways. It's said to be the longest underground conduit of freight on the Atlantic seaboard; the first example of heavy-duty railroad electrification in the United States, possibly the world; and a model example of soft-earth construction, built at a time when steam locomotives huffed and puffed through mostly rock-blasted tunnels.

"Most people who walk the streets of Baltimore don't know where the tunnel ends, how long it is or how historical it is," Courtney B. Wilson, executive director of the B&O Railroad Museum, said last night. "It really is a hidden historical asset of Baltimore."

If the tunnel's place in the public's consciousness has waned since passenger trains stopped running there four decades ago, its importance has not. Today, more freight traffic than ever before - perhaps three dozen trains a day - rumbles below Howard Street, carrying everything from hazardous chemicals to fine goods.

"It's the only freight through-route on the so-called Northeast corridor - from the South, going through Washington, coming up through Baltimore and going to Philadelphia and New York," said Herbert H. Harwood, a retired CSX Corp. official and B&O Railroad historian. "Most north-south freight goes over that route."

The tunnel was born of necessity. The B&O Railroad had a nettlesome problem by the 1880s: Its southern terminus was Camden Station, site of present-day Oriole Park, leaving no good way to link up with its new Philadelphia line that began on the east side. The railroad's imperfect solution was to ferry rail cars from Locust Point over to Canton.

In 1890, Harwood said, the B&O finally "bit the bullet" and began building the Howard Street Tunnel. But that presented a whole new set of problems. For one thing, the path from Camden Station to the tunnel's terminus at Mount Royal Station was all uphill, a steep 4.8 percent grade.

Steam engines could have chugged along, but not without spewing vast amounts of smoke and gas. And city fathers had already dealt unhappily with that scenario - trains using an earlier tunnel polluted homes and businesses through vents. The railroad was told it could not do the same to Howard Street, then a major thoroughfare and home to department stores.

That quandary gave rise to the country's first heavy-duty electrified rail service. An electric locomotive pulled trains, including the steam locomotive, from Camden Station up to Mount Royal (and later as far as Waverly). The advent of diesel in the 1950s made that unnecessary.

Even with that problem solved, there were others. "You're dealing with a completely built-up city on top of you that you can't disturb, and traffic you can't disturb," Harwood said.

But disturb it they did. One building - City College high school - fell apart. The railroad paid for a new school at what is now Howard and Centre streets.

Another problem, Harwood said, was the ground itself. "They were going through unstable soft ground a good bit of the way, as opposed to going through rock."

The tunnel was completed in 1895 at a cost of $2.2 million, Wilson said. It originally covered 7,344 feet, or about 1.4 miles. It lay 60 feet underground at its deepest, 3 feet at its shallowest.

Passenger service ended for the most part in 1958, with the last passengers riding beneath Howard in 1961. The freight never stopped rolling.

The possibility of a fire or other disaster did not go unconsidered. More than 15 years ago, officials fretted over the danger from shipments that included flammable gases, toxic chemicals and, occasionally, explosives.

In 1985, a federal transportation safety official observed: "The problem would be just getting in there to fight the fire. ... If you had an explosion, fire could shoot out both ends like a bazooka."

Over the past decade, the tunnel has gotten even busier because more Conrail freight has been routed along the CSX line, Harwood said.

And the tunnel has actually grown as it has aged. It originally ended next to Camden Station, near the present location of the light-rail tracks.

The south portal moved farther south when Interstate 395 was built and again in the 1990s to make way for the light-rail tracks. Now, the light at the end of the tunnel can be seen outside PSINet Stadium.

Even so, Harwood said, "It's basically an invisible line. Until smoke starts pouring out it."