Amtrak President Paul H. Reistrup is seeking agreement from his board of directors on future policies of the embattled rail passenger service before making a decision on whether to keep his job after July.

Reistrup said in an interview that, in any event, it is highly unlikely that he will continue as chief executive of Amtrak beyond mid-1979. At that time, Reistrup will have been Amtrak president for four years and five months, "and I don't think any person should be in this job for more than five years," he stated.

The president of the National Railroad Passenger Corp., Amtrak's formal name, is elected for one-year terms every July.

Although generally credited with providing a leadership that has led to improved Amtrak services and increased ridership in recent years, Reistrup wrote a personal letter to his directors in January expressing concern about the absence of firm policy in Washington on Amtrak's future.

The rail company has been subjected to conflicting mandates: a budget ceiling that requires cutbacks in services and a congressional order to maintain all existing trains, pending publication of a Department of Transportation report on what routes could be discontinued.

Reistrup said yesterday he met with Transportation Secretary Brock Adams recently and agreed "there would be no decision" on a resignation by Reistrup for the time being. Specifically, Reistrup said yesterday he would like to remain as chief executive at least until a new Amtrak board is fully seated after congressional approval and until the DOT route report is released.

"But something's got to give," Reistrup said of his relationship to the Amtrak board - made up of government, consumer and rail industry representatives. He said he has had one private discussion with the board about his future, that directors are considering his situation and that no future meeting has been scheduled.

Although Reistrup said he is seeking an unspecified increase in his $85,000 annual salary - low by comparison to private rail presidents but high compared with government service - he said that "is not a major" issue and that he would stay on for another full year if agreement is reached on policy direction.

Reistrup said Secretary Admas obviously would have a big role to play in such a decision but the Carter administration has been adamant in opposing increased subsidies for Amtrak. One bill before Congress, said to have general DOT support, not only would hold the subsidy flat but also would reduce Reistrup's current salary and place Amtrak under direct government control with a three-person board that includes the secretaries of Treasury and Transportation.

Congressional and industry sources said yesterday that a pay increase for Reistrup is out of the question and that they expect him to depart in July. Former Transportation Secretary Alan S. Boyd was mentioned as a potential successor to the Amtrak post.

Both Boyd and Reistrup are former top officers of the Illinois Gulf Central Railroad. Reistrup, 45, described himself as being at "mid-career," facing the necessity of moving to another position before reaching the age of 48.

Reistrup also revealed that Amtrak and the federal government expect to begin 1978 rebuilding work on the Boston-Washington rail line on April 3, assuming the absence of snowfall. He expressed particular pride that Amtrak's employes kept the trains running throughout a period of unusual snowfalls in the Midwest and Northeast. At times, we were the only thing running," he declared.