When Monet’s 1874 painting “The Railroad Bridge at Argenteuil” sold in 1988 at Christie’s in London, the $12.6 million it fetched was then the second-highest price paid at auction for a Monet. (A few months earlier his 1876 “In the Prairie” had sold for $22.5 million at Sotheby’s.)

The buyer of “The Railroad Bridge at Argenteuil” wasn’t one of the Japanese collectors who at that time were fueling the art market; they preferred Monet’s later works, particularly his “Water Lilies” or Venetian scenes. The buyer was the Nahmad family, dealers with galleries in New York and London who occasionally buy a painting and stash it away in one of their Geneva warehouses.

Now, after 20 years and a string of strong auction prices for Monets, the Nahmads are parting with “The Railroad Bridge at Argenteuil.” Christie’s plans to offer it on May 6 in New York and has given it an estimate of $35 million.

“It’s a pity, but my family wants to sell it,” David Nahmad said in a telephone interview. “The painting is a masterpiece. It’s about what the French would call charnière,” or a turning point.

The painting, which depicts a railroad bridge with two trains, was considered especially modern for its time. The trains were used by Monet as a symbol of the new industrial age.

“It’s the past and future,” Mr. Nahmad said. “There are two people walking along the water’s edge who represent the past; and representing the future is the bridge as well as the smoke pouring out of the trains.”

Guy Bennett, who oversees Christie’s Impressionist and modern art department in New York, said that Monet produced only three other comparable paintings of this subject. One is in the National Gallery of Art in Washington, one in the Philadelphia Museum of Art and one in the Musée d’Orsay in Paris.

In today’s market modern works have been bringing exceptionally high prices, and an Impressionist canvas of this rarity has not come up at auction for nearly 20 years. To secure the consignment Christie’s gave the Nahmad family a guarantee — an undisclosed amount that the auction house will pay the seller regardless of the sale’s outcome — that experts in the field say is around $34 million.

A SHOW GETS A TITLE

The next Carnegie International in Pittsburgh will be different. For the first time in its 112-year history, this survey of contemporary art will have a title: “Life on Mars: 55th Carnegie International.” And the show, which takes place every few years at the Carnegie Museum of Art, will open in the spring rather than the fall, something it hasn’t done since 1924.

“We moved it to capture the summer audience,” said Douglas Fogle, curator of contemporary art at the museum. Mr. Fogle is organizing the exhibition, which opens May 3. The last one took place in 2004.

“I have always had titles for my shows,” Mr. Fogle said. “It will feel more like a curated exhibition than a biennial.”

The first part of the title is taken from a David Bowie song about the world spinning out of control.

“It had a poetic openness to it,” Mr. Fogle said. “It’s what art does, takes you to another world.”

The 2008 show, which will be on view through Jan. 11, 2009, will include about 200 works — painting, sculpture, drawing, film, installation art, animation, video and performance art.

The artists Mr. Fogle selected range in age from 30 to 89 and include both recognized names — Doug Aitken, Vija Celmins, Rudolph Stingel, Mike Kelley — and artists who have rarely or not yet been shown in the United States.

Haegue Yang, a Korean-born installation artist who lives in Berlin, has had work on view at the Walker Art Center in Minneapolis but has not had a solo show in this country. The Thai filmmaker Apichatpong Weerasethakul has had exhibitions in Los Angeles but is relatively unknown in the art world. Maria Lassnig of Austria, the oldest artist in the show, was described by Mr. Fogle as “an artist’s artist.” Her work has been shown in this country but is far better known in Europe.

“The lesser-known artists aren’t necessarily about the shock of the new,” Mr. Fogle said, “but about a relevant mix of voices.”

ONLINE AUCTIONS

In 1999 Artnet.com gave online auctions a try. But the project proved too expensive, in part because the site didn’t have enough traffic.

“We stopped after 18 months,” said Hans Neuendorf, Artnet’s chief executive. “Back then we didn’t have the reach. Now we have two million people coming to our site every month.”

So Artnet is going to try again.

Starting Monday it will sell a selection of modern and contemporary art, prints and photographs. Unlike Sotheby’s, Christie’s and Phillips — which charge the buyer 25 percent of the first $20,000 spent, 20 percent of the next $20,000 to $500,000 and 12 percent above $500,000 — Artnet charges a flat 10 percent commission.

“We can be cheaper and faster,” Mr. Neuendorf said. “We don’t have the costs of marketing, printing catalogs or holding cocktail parties. We will be selling 24/7, offering new lots every day.”

Most offerings will be in the $10,000 to $200,000 price range.