However the maximum feasible gradient is limited by how much of a load the locomotive(s) can haul upwards.
This is because all wheels are usually connected to engine power in order to give better acceleration.
Metros and pure commuter railways often also allow steeper gradients, over 4%, for the same reason.
High-speed railways commonly allow 2.5% to 4% because the trains must be strong and have many wheels with power to reach very high speeds.
The list below is of some of the steepest gradients on adhesion railways, in order of decreasing steepness: (9.0%) Logging Museum that runs a tourist train, using a combination of steam locomotives and diesel locomotives, usually with trains consisting of 1-3 coaches in length, as well as motor cars.