With apologies to Tennessee Williams, this article could be titled "Streetcar Desirability." Trolleys can be beautiful.

In the ongoing controversy over the proposed Purple Line linking Montgomery and Prince George's counties, proponents of light rail seem to have overlooked one important consideration: the potential aesthetic benefits of a well designed trolley car system.

Most of the Purple Line debate has revolved around alignment alternatives and the choice between light and heavy rail -- that is, surface transport such as trolleys vs. subsurface transport such as subways.

Arguments have focused on costs, environmental impacts, effects of alignments on private or public property, and the effectiveness of transit in serving local and regional passengers.

Many of the arguments for light rail are based on its lower cost. But proponents also should keep in mind that an urban trolley car network can be an aesthetic asset capable of adding life to the cityscape. Well-designed light rail can be visually dynamic and colorful as well as convenient and economical.

In cities with light-rail systems, having red, yellow, green or blue trolley cars share streets with automobiles, buses, trucks, pedestrians and cyclists adds greatly to the vibrancy and animation.

This has been demonstrated by recently built light-rail systems in Salt Lake City; Portland, Ore.; and San Diego, as well as by venerable trolley networks such as Boston's. These systems not only provide necessary transit, they also improve the look of urban neighborhoods through which they pass.

It's easy to perceive trolleys as special. Running on fixed rails in streets or street medians, they sometimes seem immune to traffic congestion afflicting automobiles, taxis and buses. But like buses, trolleys allow riders to read, doze, meditate or look out large windows to observe the city. Even during rush hour, a rider at a window may feel almost privileged, as if provided with a unique, constantly moving vantage point from which to survey the dynamics of urban life.

Builders of modern light-rail systems have realized that such networks could be designed with aesthetics given high priority.

Consequently, the many visible elements associated with newer trolley networks, including trolley cars themselves, have been architecturally coordinated.

For example, the regularly spaced supports from which electric lines are suspended can become elegant, handsomely detailed structures that transcend engineering. Consequently, overhead lines powering trolleys, aligned precisely with rails below, can seem much less objectionable visually than the usual overhead utility lines strung along or across streets.

The same is true for many other repetitive elements -- platforms and canopies at trolley stops; guardrails; light poles and fixtures; benches for waiting passengers; waste receptacles; vending machines for newspapers and other items; signs; and kiosks or structures for displaying advertising and public announcements.

Fortunately, the Washington area's Metro subway system was conceived with this kind of integrated, unifying design philosophy, which accounts for its memorable aesthetic qualities. But these qualities are seen primarily in underground stations or surface stations disconnected from city streets, with little impact on the visual character of the city.

A light-rail line can do on the surface what Metrorail has done below the surface: energize the city visually and architecturally while enhancing mobility.

Yet if light rail is chosen for the Purple Line, consideration must not be limited to the design of trolley cars and the thousands of pieces of hardware within the right of way. Consideration also must be given to the use, density, geometry and character of the urban landscape flanking the line.

This raises planning and urban design questions with aesthetic implications. What land-use changes should occur near trolley stops? How and where should the trolley line spark new, more intense transit-oriented development? What effects could the Purple Line have on its surroundings, for example, as it runs along University Boulevard in Silver Spring and Langley Park?

Building the inner Purple Line presents an opportunity to do more than just add another mode of transportation. It could help to beautify streets and urban neighborhoods in sections of Montgomery and Prince George's counties that need beautification. And when the time comes to expand the transit network in other parts of the region, the Purple Line could serve as an aesthetic as well as functional model.

Roger K. Lewis is a practicing architect and a professor of architecture at the University of Maryland.