Facing a threat by the Reagan administration to cut off federal subsidies to passenger trains, Amtrak President Alan Boyd has responded with a promise that all Amtrak routes will be able to pay their own way by 1985.

Boyd contends the future of some Amtrak routes is so promising that it may be possible to persuade private investors to put up the cash to equip them with 120 m.p.h. Japanese bullet trains.

The unexpectedly optimistic view of Amtrak's future was given by Boyd in what was described as a major address Friday to the Women's Transportation Seminar.

The Amtrak boss said the federally subsidized rail passenger system plans to eliminate by 1985 those trains which cannot carry enough passengers to pay their operating expenses. Some unprofitable routes may be subsidized by more profitable runs or paid for by state and local governments under his plan.

Cutting Amtrak back to routes that will pay their own way will not mean an end to federal subsidies. Boyd acknowledged he is counting on the government to continue to pay for Amtrak's capital improvements and to finance the "overhead" cost of running the railroad's corporate headquarters and management.

Boyd said Amtrak will need $853 million this year, compared to the $613 million Reagan proposed, but the yearly subsidy should drop year by year, with $167 million fewer in constant dollars needed per year by 1985.

Boyd said Amtrak is talking with Japanese railroad builders and with "both domestic and foreign interests" to obtain private financing for Amtrak bullet trains.

He said various routes are under consideration for the high-speed, passenger-only rail lines, including some in California, Houston-Dallas, Tampa-Orlando-Miami and paralleling the current Washington-New York-Boston Northeast Corridor.

"I believe that, in carefully selected markets, we can rival Japan in the success of these trains and that we can generate enough revenue to cover operating costs and retire construction loans," Boyd said.

The so-called bullet trains operate at 120 mph between major Japanese cities.

Although the overall Japanese rail system loses money like all other passenger train systems, the bullet trains themselves show a major profit. Therefore, such a venture might be attractive to financial interests. p

But in the meantime, he said, the Reagan administration is trying to kill Amtrak just as it was beginning to thrive with full trains and new equipment.

He said budget director David Stockman "simply doesn't know what he's talking about" when he says Amtrak is running "empty rattletraps."