Questions 26 to 30 are based on the following passage.

The earlier type of suburb, which was most dependent on the railroad, had a special advantage that could be fully appreciated only after it had disappeared. These suburbs, spread out along a railroad line, were discontinuous and properly spaced; and without the aid of legislation (法规) they were limited in population as well as area; for the biggest rarely held as many as ten thousand people, and under five thousand was more usual. In 1950, for example, Bronxville, New York, a typical upper-class suburb, had 6,778 people, while Riverside, Illinois, founded as early as 1869, had only 9.153.

The size and scale of the suburb, that of neighborhood unit, was not entirely the result of its open planning, which favored low densities. Being served by a railroad line, with station stops from three to five miles apart, there was a natural limit to the spread of any particular community. House had to be sited “within easy walking distance of the railroad station,” as some old residents would point out; and only those wealthy enough to afford a horse and a carriage dared to penetrate farther into the open country.

Through its spaced station stops, the railroad suburb was at first kept from spreading or excessively increasing in numbers, for a natural greenbelt, often still under cultivation as park, gardens, remained between the suburbs and increased the available recreation area. Occasionally, in a few happy areas like Westchester, between 1915 and 1935 a parkway, like the Bronx River parkway, accompanied by continuous strip of park for pedestrian (散步的人) use, not yet overrun by a constant stream of traffic, added to the perfection of the whole sub pattern. Whatever one might say of the social disadvantages this was in many ways a perfect physical environment. But it lasted less than a generation.

What was the special advantage of the old type of suburb?