Task: Question Answering
Input (English): Passage: F. Nelson Blount was killed when his private airplane hit a tree during an emergency landing, in Marlboro, New Hampshire, on August 31, 1967. By that time a good deal of Blount's collection was controlled by the Steamtown Foundation and had been relocated to Bellows Falls. Blount owned several corporations and one, the Green Mountain Railroad (GMRC), controlled the tracks that lay between Walpole, Bellows Falls and Chester, Vermont, which Steamtown was to use for its excursions. When Blount died most of the controlling stock of the GMRC was transferred to the president of the railroad, Robert Adams.
Throughout its tenure in Vermont, Steamtown provided several types of excursions, primarily in the summer and during the peak foliage season of the autumn.  Occasionally, these trips would be lengthy, like one that ran from Boston to Montreal, or those that ran between Bellows Falls and Rutland, Vermont.  On a daily basis the excursions ran from Riverside station in Bellows Falls to Chester depot.  The cost of the trip, which in 1977 was $5.75 for an adult and $2.95 for a child, was combined with entrance into the museum, which was the grounds of Riverside station. The station was located about 2 miles (3 km) outside of town and was situated on the bank of the Connecticut River.  One newspaper travel writer, Bill Rice, described the 13-mile (21 km) trip from Riverside to Chester: "The trip to Chester affords a beautiful view of unspoiled Vermont countryside-covered bridges, vintage farms with grazing livestock and cornfield and a winding river with a deep gorge and picturesque waterfall." The river that Rice referred to was the Williams River, which crossed the route of the train seven times.  The waterfall was at Brockway Mills Gorge and was seen from a bridge 100 feet (30 m) above the gorge.  Rice also said that at the time he was writing, 1977, Steamtown had the largest collection of steam locomotives in the world.In 1971, the Board of Health of Vermont issued a waiver to the GMRC for Vermont's air pollution regulations.  The waiver permitted the operation of steam locomotive excursions between Steamtown's Riverside station at Bellows Falls, and Chester depot.  In 1974, as the state of Vermont prepared for its celebration of the country's bicentenary, in which the Steamtown excursion featured prominently, the subject of the air pollution regulations came up again.  The tourist attraction was operating on temporary permits that allowed it to operate excursions in Vermont.  By 1976, the relationship between Steamtown and GMRC deteriorated as the two organizations fought over maintenance of the tracks, which were owned by the state of Vermont. 
Question: What did the waiver allow that was issued by the Board of Health of Vermont to GMRC?
Output (English): the operation of steam locomotive excursions.