THOUGH FEDERAL subsidies have long supported almost every form of transportation in this country, from jogging to jetting, the Reagan administration is now seeking to eliminate all federal assistance to Amtrak. This move would surely kill intercity passenger rail service, squander a huge investment that is just beginning to pay off and create new transportation pressures all up and down the Northeast Corridor. One would like to believe that rational members of Congress will resist this penny-wise and inconsistent transportation proposal.

But the administration is pushing hard for a cutoff.

The administration contends that Amtrak services an insignificant 2 percent of intercity passenger trips nationally and that private rail companies will swoop right in and pick up Amtrak's busiest routes, such as the one between Washington and New York. What if they don't? How much would taxpayers wind up underwriting in the way of road money and federal support for additional airline service?

Another administration argument is that Amtrak's Northeast Corridor service is serving mostly higher-income people: about 55 percent of the riders are paid more than $30,000 annually. Does this mean that people with incomes under this level don't depend on Amtrak and that they all can and do stick to bus service? That's hard to believe -- and hard to confirm.

But as long as people are tossing about all sorts of numbers, Amtrak officials have some too. They argue that if all federal support for such services as air traffic control were totaled, subsidies for air travel would be much higher than what Amtrak is receiving. They note that capital investments totaling $5.2 billion are just beginning to pay off in better service and economy. And dismantling the system, if it came to that, would cost the government billions over the next six years.

On a local note that is nevertheless near and incredibly dear to the pockets of taxpayers from coast to coast, the federal government does happen to be financing plans to redo Union Station. What for? "Maybe you should have alternative plans for Union Station," says Rep. Silvio Conte of Massachusetts, who opposes the proposal to eliminate Amtrak's subsidies. "Put in some stalls and hay racks."