A trail on a former rail bed feels different underfoot from any other long path. The gradient barely changes, the curves are long, and the surface is consistent for miles.
Locomotives set the maximum gradient
A train's pulling power depends on friction between steel wheels and steel rail, which is low. A modest gradient quickly consumes the available tractive effort.
Surveyors therefore held mainline grades to a small percentage, accepting longer routes rather than steeper ones.
That constraint is why a rail bed climbs a valley almost imperceptibly where a road alongside it takes a direct and much steeper line.
Curves had to be broad
A rigid wheelbase cannot follow a tight radius without excessive wear and speed restriction, so curves were laid out generously and eased into with transitions.
The result is an alignment with no sharp corners, which is why sight lines on a rail trail are long and the walking rhythm is uninterrupted.
It also means the path rarely deviates for a minor obstacle, since the survey chose the line before the obstacles were dealt with.
Engineering structures came with the route
Holding a gradient across broken country required cuttings, embankments, bridges and tunnels, all of which were built to carry heavy loads.
Those structures survive well and are the reason a rail trail crosses rivers and ravines that would otherwise force a long detour.
Tunnels in particular are the feature walkers notice most, and they are also the maintenance liability that most often closes a section.
Drainage was designed in
A rail bed was built with ballast, ditches and culverts to keep water away from the formation, because a waterlogged track bed fails under load.
That drainage is why these paths stay usable in wet conditions when an adjacent field path is impassable.
Where culverts have collapsed, the same alignment floods badly, which is usually the reason a rail trail has an unexplained closed section.
What the format is good and bad at
The gentle gradient makes these routes suitable for walkers who cannot manage hills, and the consistent surface suits wheeled use.
The same qualities make them monotonous over long distances, since the alignment avoids the ridges and summits that give a walk its variety.
They work best as a link between more interesting terrain, or as a way to cover distance efficiently at the start or end of a longer route.