Metrorail ridership may be spreading like wildfire. But what if fire ever spread through Metrorail?

What contingency plans exist? Are escape routes accessible? What would burn inside a Metro car? Can enough emergency equipment get close enough to the trouble quickly enough?

In short, could there be a disaster?

Very unlikely, say safety officials for Metro and for the six fire departments that have Metro running through their jurisdictions, or soon will.

Not only was Metro built with fire safety in mind, officials said, but extra fire protection has been added on the basis of the two years of Metrorail experience.

If a fire occurred underground, smoke detectors would alert Metro headquarters. The detectors are so sensitive that they sometimes sound when the humidity goes up.

In most cases, fire apparatus would be on the scene within two minutes. Feeding hose through fan shafts, ventilation shafts or emergency exits, firefighters could tap into a track-level source of water every 200 feet throughout the system.

Evacuation is a last resort in contingency planning, regardless of the nature of an emergency. But officials said evacuation would be easier and safer than in any other subway system in North America.

Metro has emergency exits at least every 2,500 feet throughout its system, or more than 100 in all. And since escalators in stations are designed to move large numbers of people quickly, clearing a station in a short time "should never be a problem" as long as electric power is available, said Lawrence Engleman, Metro's fire protection coordinator.

But prevention, not cure, is Metro's basic approach to trouble. And sophisticated electronics are the key to prevention.

Because trains are started, stopped and kept separated by a computer at Metro's downtown Washington headquarters, the chances of a collision are almost negligible, officials said. In addition, the engineer of every train has a direct radio link to headquarters, unlike many systems, officials said.

In any underground emergency, whether it involves a fire or not, Metro contingency plans call for the engineer of a train stalled in a tunnel to first try to reach a station.

If he cannot, he would consult by radio with Metro headquarters, which would send a second train to try to push or pull the first train to the nearest station. Only if that failed would evacuation be considered.

"We want to do everything we can not to have people walking through a tunnel," said Engleman. It hasn't happened yet, he added.

On the above-ground portions of Metro, the same policy applies. Evacuation is a last resort. In addition, Metro rules forbid an engineer whose train is in any kind of trouble above ground from entering a tunnel to reach a station.

If passengers ever did have to evacuate a train stopped in a tunnel, they would walk to the nearest exit along a concrete cat walk 22 inches above track level. Every fourth light in every tunnel is an emergency light and would stay lit even if normal power were out. No emergency exit door requires more than 50 pounds of force to open, and each is checked regularly for flaws. No emergency exit is ever locked.

Metro subway cars are well-enough sealed that fire outside a car would not begin to penetrate it in less than 20 minutes, said Batallion Chief Hugh Clarke, the D.C. Fire Department's Metro liaison officer. Furthermore, seals against smoke in Metro cars are "95 percent perfect," said Engleman. And the only remotely flammable material in Metro tunnels is electrical wiring, he said.

Still, although most are hardly worthy of the name, there have been more than 100 fires in Metro's two years of service.

About a dozen have required a response from a fire department, although there have been no injuries and little damage. Several dozen other fires, all involving such things as burning newspaper or carelessly discarded cigarettes, have been extinguished by kiosk attendants or passengers.

Ten of the fires have occurred in the same place - a portion of the braking system in the undercarriage of a car known as a resistor bank.

To date, the resistor bank episodes have produced smoke, but no flames.They have been caused when friction and heat built up on a synthetic filter.

The episodes are neither dangerous nor costly, according to officials. But they have caused delays.Metro routinely cuts off third-rail power while resistor bank smoke is extinguished. Service has been interrupted for as long as an hour.

Long-range prevention of these incidents "will require a design change," said Engleman. Metro officials are studying a filter made of a more heat-resistant material.

The three other most serious fires have involved a smoking filter in a ventilating shaft near the Union Station platform last April, a collection of cleaning fluids that caught fire in a storage closet at the Pentagon City station last winter and a battery charger that caught fire last March in a power room at the Federal Triangle station.

There were no injuries in any of the three and little damage or interruption of service. But Metro is in the process of fitting its 80 storage rooms with sprinkler systems just the same.

Metro might not have been so fire-proof if two key decisions had not been made in 1975.

In the original design of the system, firefighters would have been able to reach a track-level water source (known as a standpipe) only via fan shafts. These are several hundred yards apart in many cases.

But at the urging of fire officials, Metro modified its design to provide standpipes every 200 feet throughout the system.Underground standpipes can be reached directly from above in most cases. Meanwhile, fire officials in some jurisdictions ordered diagrams of the locations of standpipes to be carried on fire trucks.

Officials also junked the original polyurethane seats in Metro cars when a test by the National Bureau of Standards revealed that the seats were flammable. They have been replaced by seats made of neoprene, which burns only in intense heat.

At the same time, armrests beside several seats were removed so that any fire would have a harder time climbing the walls of a car. The car walls themselves were covered with a plastic more resistant to flames than the material originally planned.

The Bureau of Standards tests were ordered after a Washington boy, who had apparently fallen asleep, died in a fire on a Metrobus. Metrobus seats were made of the same material as the seats originally planned for Metrorail. Bus seats were also changed as a result of the Bureau of Standards tests.

According to Engleman, the two 1975 Metrorail modifications cost about $2 million. "The board was willing to spend the money," Engleman said. "We've gone the extra mile. Now, I don't think we could do a whole lot better.

"The sense of management now is that they're committed to safety, regardless of the original design and as long as the cost is within reason."