Plea bargaining is one of the great abominations of the American criminal justice system.

In fact, it is a double abomination, because it lets people guilty of egregious crimes get off with minor punishment and it pressures innocent people to plead guilty to crimes they never committed out of fear that they will get harsh punishment if they fight for their rights.

But this outrage called plea bargaining survives because most prosecutors don't want to work too hard, and judges don't want their dockets clogged with cases that can be disposed of in out-of-court deals.

One of the worst examples of plea bargaining just occurred in Maryland, where former Conrail engineer Rick L. Gates was allowed to plead guilty to a ''misdemeanor,'' or a single count of ''manslaughter by locomotive,'' after admitting that he smoked marijuana before driving three diesel locomotives into the worst train wreck in Amtrak history.

Sixteen passengers were killed and 176 were injured on a Washington-to-Boston train that was going more than 105 miles per hour when it plowed into the rear of the engines that Gates had taken onto the tracks despite warnings to slow down, and then to stop.

Under his plea bargain, Gates may have to stay in prison for only 20 months and pay a fine of $1,000 for all the carnage and human suffering he caused.

Anne Johnson, whose 20-year-old daughter was killed in the crash, says she ''wanted to throw up'' when she learned of the relative wrist-slap that Gates got in his plea bargain.

I usually am not overwhelmed by the complaints of the relatives of crime victims, because many wouldn't be satisfied until they saw the accused hanging from a gallows in Central Square. But if relatives ever had a reason to complain, it is in this case. Ponder a few facts:

Baltimore County State's Attorney Sandra O'Connor already had granted immunity to Conrail brakeman Edward Cromwell for his testimony that he and Gates had smoked marijuana just before their locomotives ran through ''slow down'' and ''stop'' signals onto the main northbound Amtrak tracks.

Even with Cromwell's testimony, O'Connor sought a plea bargain out of fear that the state could not prove that Gates was ''impaired'' by the marijuana.

O'Connor gave the excuse that she plea bargained with Gates to avoid an emotionally wrenching trial, and to ''make the proceedings easier on the victims.'' Now, if the plea-bargain deal wouldn't make a relative throw up, that excuse for it ought to.

Roger A. Horn, whose 16-year-old daughter, Ceres, died in the wreck, looked at the carnage, then at the plea bargain, and said bitterly that Gates' crime was treated as though ''it's not as serious as stealing a refrigerator or a television set.'' Horn spoke the truth.

Plea bargaining has long upset millions of Americans and divided the legal profession. There was a flood of protest in 1973 when Vice President Spiro Agnew was allowed to plead ''no contest'' to charges of income tax evasion, his punishment limited to three years' unsupervised probation and a fine of $10,000.

Plea-bargain deals with assorted Watergate culprits added to the public outrage.

But plea bargaining has been defended by many lawyers and judges, including former chief justice Warren E. Burger, who wrote in 1971 that it ''is an essential component of the administration of justice . . . and properly administered, is to be encouraged.''

I say it can never be ''properly administered,'' is an open invitation to all kinds of injustices and should be abolished.

The Amtrak tragedy is warning enough that we ought to pay enough prosecutors to prosecute, and an ample number of judges to judge, and not resort to the penny-pinching expedient of out-of-sight bargains and deals.