Rep. Cardiss Collins (D-Ill.) canceled an oversight hearing this week into what she described as evidence of "theft and fraud" at Amtrak after the president of the passenger rail service and a railroad union official urged her to delay it.

Collins had announced in a sharply worded news release last week that she would "thoroughly investigate" allegations involving an Amtrak "slush fund" and a "cover-up" by top officials. But last Monday, two days before witnesses were to appear before her Government Operations subcommittee on transportation, Collins said there would be no hearing until uncertainty over Amtrak's budget is resolved.

Amtrak President W. Graham Claytor Jr. asked Collins to postpone the hearing in a letter last week, saying he is working full time to defeat President Reagan's proposal to end federal subsidies to Amtrak. Collins agreed to drop the hearing when Jim Snyder, legislative director of the United Transportation Union, which represents about 10,000 Amtrak employes, also urged her to do so.

A Collins aide said she hopes to reschedule the hearing this spring, but wanted to avoid jeopardizing efforts to continue a $684 million federal subsidy to Amtrak next year.

"I will not give ammunition to those in the White House who are plotting to kill Amtrak and place thousands more of Amtrak's union employes on the street," Collins said in a statement. " . . . The cover-up I found will unfortunately reduce Amtrak's credibility as we in Congress try to save it."

Claytor asked Collins in a Feb. 26 letter if she would "have your hearing rescheduled for sometime later this year." He said he was busy meeting "with as many members of Congress as I can possibly see in the coming weeks in order to explain the consequences of providing no federal funds for Amtrak."

Collins first wrote Claytor that she would not defer the hearing but changed her mind this week, Amtrak spokesman Sue Martin said.

Snyder said he asked Collins this week "just to delay or postpone the hearing until after we see where we're going on Amtrak. We just thought the timing on it was not good . . . . We can just be in so many places at one time." In her release, Collins said that Amtrak officials in Chicago have used an emergency equipment fund to "purchase tools for their personal collections, obtain paint and replacement parts for their private autos and place unauthorized stereos in company cars."

Collins said some officials also have stolen Amtrak equipment, and that an investigation by Amtrak police was "astonishingly inept."

"Amtrak underreported the magnitude of the crimes and the involvement of management officials to local prosecutors and to this subcommittee in order to protect them," her release said. " . . . Although at least a half-dozen management employes were involved or benefited from the scheme, none were punished . . . . "

Martin said that Amtrak "discovered the problem" and fired some employes involved. "We feel confident that we handled it fairly and fully, and that when Collins does hold her hearing nothing to the contrary will emerge," she said