The 27,000 people who work in the Pentagon, including jut-jawed generals and assorted personages of power, must all commute twice a day - all, that is, except for one man.

That man, with the unassuming title of Navy journalist second class, simply walks across some grass and climbs abroad his own private railroad car.

The car, tastefully decorated with prerevolutionary Russian ethcings and red velvet curtains, rolled up to within a quarter-mile of the free world's defense headquarters when Tyler Robbins, 30, decided to take some temporary employment with the Navy.

The former New York Central private car, equipped with four bathrooms, a nickel sink and a brass shower head, pulled into Washington on May 8.

Robbins said yesterday he has tried to keep the 82-foot-long car, which is of the sort that brings tears to rich people's eyes, a secret. The car, however, can be seen from airplanes that fly over the Potomac River to and from National Airport and from cars on the Virginia-bound lanes of the 14th Street Bridge.

Sooner or later, Robbins admits, somebody had to spot the Pentagon's only one-unit campground.

"The trouble is that everyone in the government is very, very concerned. You've got the GSA investigation going on. The car was placed there (on a side track that Robbins rents from Conrall) with the understanding that it would be removed if a problem should come up," Robbins said.

Assuming that the Pentagon, where many people are very concerned about many things, might object to his apartment on wheels, Robbins said he checked "with every possible agency" beginning over a year before the car arrived here.

"I checked with more than 25 people to make sure that no one would be upset," Robbins said.No one was. It seemed the Pentagon had neglected to draw up regulations concerning employes who want to bring their railroad cars to work with them.

Robbins' car, which can sleep eight and was once staffed with a steward, a porter and a railroad chef, is valued at about $25,000. It can be pulled anywhere Robbins wants to go, provided he is willing to pay between $1 and $1.50 a mile and make travel reservations several months in advance.

"It is a dinosaur," Robbins said."I honestly don't know why I have it."

He bought the 30-year-old dinosaur in 1972 when it was cripped in a scrap yard in his hometown, Cleveland, Ohio. It cost $9,000, and Robbins paid Penn Central train fixers $4,000 to make some major repairs. Then Robbins said he spent another $10,000 to make the car comfortable.

Edwin P. Alexander, a train expert from Yardley, Pa., who has written 10 books about trains, said yesterday that Robbins is one of a dwindling few in the country who keep their private cars ready to travel.

"Most of the cars are anchored somewhere and that is that," Alexander said.

Robbins said he plans to pull out Nov. 15, when his job with the Navy is ended. He doesn't want fanfare before then and he wants to roll out of town quietly.