BALTIMORE, FEB. 18 -- Local authorities will give federal prosecutors evidence that a railroad union official may have obstructed a federal investigation of the fatal Conrail-Amtrak wreck near here last year by failing to report he knew that two Conrail crew members had smoked marijuana just before the crash.

The action comes just after Baltimore County prosecutors negotiated a manslaughter guilty plea by former Conrail engineer Ricky L. Gates in the fiery crash that took 16 lives, the worst in Amtrak history. Prosecutors said today they are preparing information that local United Transportation Union representative H.A. (Bud) Daugherty learned from Gates and fellow Conrail crewman Edward Cromwell shortly after the disaster that the two men smoked parts of a marijuana cigarette minutes before the collision on Jan. 4, 1987.

Daugherty was present three days later at a preliminary investigative hearing by the National Transportation Safety Board in Baltimore, according to county prosecutors, but apparently said nothing when both Gates and Cromwell denied using drugs on the day of the accident.

Federal prosecutors, who have been monitoring the county case against Gates since last year, said today they would accept the information from county officials but cautioned that it would have to be analyzed carefully to determine if any federal criminal laws are applicable.

Also, an attorney for Daugherty's union in Washington said the admission of marijuana use by Gates and Cromwell to a union representative may be protected from disclosure by rules of confidentiality, similar to those of a physician and patient or priest and penitent.

"If a union official accepts information in confidence, he is under some fiduciary duty" to maintain the confidence, said Lawrence M. Mann, attorney for the Railway Labor Executives Association.

On the other hand, Mann said, "We're talking here about a lot of deaths and a lot of infractions {of railroad rules} . . . . That's where the tough question is."

Mann said he was not aware of Daugherty's knowledge of the marijuana use until it was disclosed at at the hearing this week at which Gates pleaded guilty. "If I had known, I certainly would have made a strong suggestion that we {the union} come forward with everything."

Daugherty, a representative of Local 454 of the United Transportation Union in Baltimore, could not be reach for comment.

Gates, 33, pleaded guilty Tuesday in Baltimore County Circuit Court to one count of manslaughter by locomotive. He faces a possible maximum penalty of five years in prison and $1,000 in fines. Cromwell was granted immunity and cooperated with prosecutors in the case. He is not expected to be prosecuted.

According to a statement of facts filed in the case, Gates was at the controls of a string of three Conrail diesel engines when they passed a series of "slow" and "stop" signals and rolled across a closed switch into the path of a speeding Washington-to-Boston Amtrak train with more than 600 passengers.

According to the statement of facts, which was signed by Gates, Gates and Cromwell shared a hand-rolled marijuana cigarette in the minutes before the crash, taking about three drags each as the train traveled at 60 miles an hour.

Drug experts said later that both Gates and Cromwell had traces of the illicit drug in their blood and urine, but the experts could not say whether they were impaired by it.

Shortly after the accident, according to the statement of facts, Gates told Daugherty about the marijuana. Daugherty, acting as union representative for Gates and Cromwell, then appeared at the preliminary safety board inquiry into the accident a day or so later but was silent when the two trainmen denied using drugs, according to county prosecutors.

First Assistant U.S. Attorney Gary P. Jordan, Maryland's second highest ranking federal prosecutor, said today he had not yet seen the information on Daugherty but said "hypothetically" that several federal criminal laws "might come into play" including obstruction of a federal investigation.

Jordan said he is not aware of any law that protects confidential admissions by a union member to a union official "in the criminal sphere." The law, he said, "does not allow people to invent confidential relationships . . . . There has to be some legally recognized privilege, such as between a lawyer and his client."