Thousands of railroad workers, many of them on the track to unemployment, marched on Capitol Hill yesterday to protest planned cutbacks in federal rail subsidies.

The demonstrators came from all over the United States, from cities and towns served by the federally subsidized freight (Conrail) and passenger (Amtrak) rail systems that the Reagan administration says are draining billions of dollars from the U.S. Treasury.

The protesters and their supporters -- between 10,000 and 18,000 persons, depending on whether you believe the District of Columbia police or the unions involved -- came mostly from the Northeast. Their presence here disrupted rail and passenger service in places such as New York and Philadelphia, where Conrail and Amtrak officials said they were "experiencing some difficulties due to a lack of personnel," as one put it.

If the service disruptions mattered to the assembled masses on the grounds of the Capitol's West Front, not many seemed to show it. They had their signs: "Hey, Dad, Mom Supports Us. What Do You Do?" and "We Want Work, Not Welfare." They had their fears: no trains, no jobs.

The issues were clear to the demonstrators and their leaders.

One, the Reagan administration wants to get the government out of the railroad business by cutting federal spending on trains by $5.3 billion over the next five years. One approach to this objective is to end all federal aid to Conrail and rural freight lines.

Conrail, a short-haul, terminal-intensive railroad, now has about 80,000 employes. Union officials have argued, as they did again yesterday, that the Reagan plan would eliminate 40,000 to 50,000 of those jobs.

Two, Amtrak has asked the government for $716 million in operating subsidies for 1982 and another $254 million for equipment and capital improvements -- a total of $970 million. The Reagan administration says that's too much to pour into the constantly troubled passenger service and has offered $613 million instead. This plan won Senate Commerce Committee approval yesterday by a 10-to-4 vote.

Union officials argue that the difference means curtailing all Amtrak service outside of the Northeast corridor and putting about 20,000 Amtrak employes out of work.

Three, leaders of the Brotherhood of Railway and Airline Clerks, the Railway Labor Executives' Association, the United Transportation Union and related labor groups say that any breakup of Conrail and loss of Amtrak jobs would seriously undermine the Railroad Retirement System, which they say already is paying out more in pensions than it is taking in from working contributors.

First came Fred J. Kroll, the BRAC's international president, who told the cheering throng that Reagan Inc. is "out to destroy Conrail and Amtrak" and to "substitute a scam and a fraud upon the railroad workers." Many of these workers "were taken in by Reagan's rhetoric during the campaign and, like other people who were taken in, voted for him," Kroll later conceded in an interview.

AFL-CIO President Lane Kirkland, in an uncharacteristically forceful delivery, also spoke of support for Reagan among workers. During the recent presidential campaign, conservative politicians attempted to woo organized workers by telling them their leaders were out of step with their feelings, Kirkland said.

He said yesterday's demonstration proves that "it is not the labor movement, but too many politicians who have lost touch with the aspirations of the American people."