A helix, as it is traditionally built on a model railroad, doesn't resemble many prototypical track arrangements. But these spirals aren't completely reserved for model trains. Railroad engineers have come up with some pretty awesome means of gaining elevation while minimizing the grade.
As a general rule, model or prototype, lengthen the run and you minimize the grade. In addition to following the contours of the land, curves also add distance to the run and reduce the grade. One of the most famous early examples of this concept is the Pennsylvania Railroad's (now Norfolk Southern's) Horseshoe Curve just west of Altoona, Pennsylvania. Completed in 1854, this graceful arc crosses two valleys and utilizes three ridges to gain elevation while keeping the grade to about 1.8%. In addition to helping trains get uphill, all of these curves also add friction which helps control the speed of downhill trains.
The most famous prototypical helix applications in North America are Canada's Spiral Tunnels and California's Tehachapi Loop. The latter is a complete loop which highlights the serpentine grade from the Pacific coast to the summit of the Tehachapi Mountains in Central California on today's Union Pacific and BNSF mainline. From a vantage point on the hillside above, spectators can watch a train climb over itself as it negotiates the loop.
The Spiral Tunnels actually feature a pair of loops which, much like many of their model brethren, are buried in the mountains! The engineering and construction resources needed to engineer these spectacular curved and graded tunnels in 1909 are mind-boggling even today.
All of these extreme engineering examples are the most famous highlights of extended twisting and demanding grades. There is ample prototype for making a helix only one part of the climb on your railroad, using more traditional grades on either side to complete the run.
