Just outside the Desert Room at Phipps Conservatory and Botanical Gardens is another Western landscape, where tiny trains chug up and down a snow-topped mountain, by stands of pine and fir trees.

The garden railroad has returned to Phipps, and this time, it's outdoors. For nearly eight weeks, a team of more than 30 volunteers and staff have built, painted and planted a display that allows visitors to see the trains, tracks and plants up close.

With "Chihuly at Phipps: Gardens & Glass" still in full swing, placing the railroad inside the conservatory was not an option. Instead, Phipps' staff chose the green roof next to the Desert Room and Japanese Courtyard Garden.

"We wanted visitors to see the trains up close, and this space allows them to walk all the way around the display," said executive director Richard V. Piacentini.

Phipps enlisted the help of volunteers from the Pittsburgh Garden Railway Society. Once the 50-by-25-foot wooden platform was built, society members stepped in to design and construct the mountains and valleys. Starting with a wooden framework covered with chicken wire, volunteers added mortar-soaked burlap followed by 1 inch of hand-applied mortar that was then spray-painted and textured. The Southwest theme came into view with rocky mountains and formations, tunnels, a turn-of-the-century gold mine and even a miniature Mesa Verde.

The garden railroad and its 350 feet of track carry three trains -- an LGB Mogul freight train, passenger steam locomotive and mining train. Sound effects and lights add whimsy to the display, which features adobe buildings, animated prairie dogs, miniature people and cattle.

Because this is Phipps, the plants are key features, too. Phipps' horticulturists added soil and planted small-scale versions of various evergreens, succulents and perennials.

"We chose plants that fit in with the theme and scale of the buildings and engines, as well as tolerate the increased cold and wind exposure that comes with an elevated setting," said Megan Fifarek, Phipps' greenhouse production foreman.

The horticulturists chose seedlings because they have natural variation and therefore look more natural. Also, seedlings can better tolerate weather, pests and other problems, requiring less human intervention and making them more sustainable.

Curt Pesanka, Phipps' indoor display foreman, said the plantings are clustered in two primary areas.

"On the eastern side of the display, we wanted an evergreen environment where the plantings replicate a forest-like climate. We used a well-drained mix of soil and various rock amendments to help eliminate any chance of frost."

The evergreens include Norway spruce (Picea abies), Fraser fir (Abies fraseri), Canadian hemlock (Tsuga canadensis) and Eastern white pine (Pinus strobus).

The western side of the display is an arid climate with succulent plants, varieties of thyme and junipers. The soil mix is more porous with larger aggregate rock cover, and plant varieties were chosen to resemble desert-scrub lowlands. These include hens and chicks (Sempervivum tectorum) and 'Lemon Frost' and 'Silver Posie' thyme.

Mr. Pesanka and Ms. Fifarek stressed the importance of choosing plants that can withstand the elements. One example is creeping raspberry (Rubus pentalobus), used as an edger along the eastern side. This hardy, fast-growing evergreen groundcover spreads 3 to 6 feet in all directions by runners. It offers four seasons of color and tolerates extreme light, heat and dryness.

"The evergreens won't need much maintenance this winter. We'll trim them to keep their shape and size, but the significant rains we had recently gave the roots plenty to work with for the cold months ahead," Ms. Fifarek said.

Once the plants are in place and the trains are running, the next challenge is to keep them that way.

"Wind will blow the trains right off the track," said Dave Bodnar, one of the Railway Society volunteers who work with Phipps staff to keep the tracks free of snow, rain and ice.

The display is meant to be interactive. Visitors can hear an explosion at the gold mine and see prairie dogs pop out of the ground. Although the group has already logged more than 1,200 man-hours, they're not finished.

"We're going to be adding things to it and changing things over time," Mr. Bodnar says.

Phipps Garden Railroad will run throughout the holidays and into the spring, weather permitting. The exhibit will change to a chocolate theme with Phipps' summer show. For more photos, track plans and other information, go to www.pittsburghgardentrains.com or www.phipps.conservatory.org.