Fifteen years ago police patrols in south Arlington County were rousting hobos from near the railroad tracks while beat officers in Rosslyn kept busy breaking up drunken brawls.

"A typical detail for the officer was to block Route 1 when the trains needed to cross it," Sgt. Clyde Hall, of the Arlington County Police Department, said recently.

But today Rosslyn and Crystal City, with their multitude of high-rise office buildings and streams of commuters, are challenging the suburban Arlington police department with new and more sophisticated crimes.

The developments have cleaned out the pawn shops and cheap bars in Rosslyn and the desolate areas of Crystal City and thus eliminated much of the street crime, according to Lt. Robert Minnich of the department's criminal investigation division. Instead, Rosslyn and Crystal City are plagued with more property crimes, especially larcenies and burglaries, he said.

"Crime was kind of small time then," he said. "Back then a robbery call on the police radio was something that excited the whole crew. But we have robbery calls constantly."

The number of larcenies reported in Rosslyn has remained fairly steady during the last four years, averaging around 750 cases a year. But in Crystal City, the number of reports rose from an average of nearly 400 in 1975 and 1976 to 657 last year. By the end of June, almost 500 had been reported this year.

The larcenies and burglaries range from purses stolen from desk drawers to the removal of office machines worth several hundred dollars. They occur night and day, in offices, hallways and underground parking garages, police said.

Much of the problem with crime in Rosslyn and Crystal City is caused by poor security in the high-rise buildings. Capt. William A. Allen said. Many of the buildings are accessible night and day and the inside doors are often easy to force open, he said.

Both Crystal City and Rosslyn have mazes of underground parking that also are prime targets for thieves, police said. Many times the thieves park their cars in these garages, which are generally easily accessible, and use elevators to get into the buildings, police said. Cars parked in the garages are also often burglarized.

Most of the thefts are never solved, according to police. Part of the problem is that patrol officers riding around in squad cars cannot effectively prevent crimes hidden from their view in the high-rise buildings, police officials admit.

Arlington Police Chief William K. Stover said the county needs some sort of tactical unit to supplement the patrol officer. But manpower constraints are limiting his force, he said.

"(The beat officer) can't go in and police the buildings," Stover said. "The only countermeasure we can take is through our crime resistance effort."

Since Sept. 1 the police have been reviewing all site plans and final building plans for construction in the county and making recommendations for adequate security, according to Lt. John Quade, head of the department crime resistance section. Any improvements recommended by the police are not mandatory, however, he said.

Although police officials have high hopes for the new building review process, most of the area in Rosslyn and Crystal City has already been developed and the existing buildings need better security, they said. Many of the improvements are inexpensive and easy to install. Police officials said they are trying to encourage building managers to contact the police department for a survey of their building and recommendations.