Dunderberg, "Thunder Mountain", is located on the west shore of the Hudson River opposite Peekskill, about 35 miles north of the city of New York. It stands at the head of the wide section of river called Haverstraw Bay, and the river bend at Dunderberg marks the beginning of an upriver journey into the scenic Hudson Highlands.
Because of the steep and rocky terrain, the mountain escaped development up until the spiral railway project. There were some iron mines up in the hills back of Dunderberg during the colonial period, and some logging was done, but it was still a scenic woodland in 1889 when it was discovered by Henry J Mumford. Mumford and his brother operated a similar scenic railway in Mauch Chunk, Pennsylvania, with two inclined planes and coasting runs, which had been converted from a mining railroad. The "Switch Back" railroad was a popular tourist attraction in the second half of the century, and Mumford believed that a similar attraction closer to the eastern cities would be even more popular. Dunderberg was ideal because not only was it near New York but a trip could be combined with an excursion ride on the Hudson River steamboats.
It's not known exactly what happened to the Dunderberg Spiral Railway. Construction began at multiple locations in the spring of 1890, and about a million dollars was expended on labor and materials for about a year. The amount of work that was accomplished in that time, with nothing but hand tools and muscles, is a marvel to see. Probably the investors' money ran out, and all was lost. No further work was done. The clearing around the inclined planes was still visible from a distance twenty years later, but by now, a hundred years on, trees grow on the grade and small streams have washed through. Still, as Park official Gordon J Thompson reported in 1978, "with noted exceptions, the grade is in excellent condition and could be envied by the county's operating railroads".