Minimum Train Rollaway Gradient?

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For any type of train or railcar on any type of railway, be it mainline freight or light rail, what are typically the respective minimum rollaway gradients when all brakes are fully released and no power is applied, in the absence of any external forces besides gravity and friction such as wind, seismic activity, or other objects pushing or pulling on it? If you have any personal experience, please also comment.

Most mainline railroads (more accurately railways) and metros in the world limit the exceptional maximum gradient to 1% = 1:100 at station platforms, with 0.5% = 1:200 being the normal maximum, 0.35% = 1:285.7̅1̅4̅2̅8̅5̅ being the preferred maximum (which is also the standard minimum in underground and aerial structures for drainage purposes for the WMATA Metrorail), and 0.3% = 1:333.3̅ being the normal minimum in underground and aerial structures (including stations) for drainage purposes for some of the other metro systems. Most light rails and monorails limit the exceptional maximum gradient to 2% = 1:50 at station platforms, with 1% = 1:100 being the normal maximum, 0.5% = 1:200 being the preferred maximum (which is also the standard minimum for Sound Transit Link), and 0.3% = 1:333.3 being the normal minimum in underground and aerial structures (including stations) for drainage purposes.

Most mainline railways limit the exceptional maximum gradient in parking and coupling/decoupling areas to 0.25% = 1:400, with the normal maximum being 0.2% = 1:500 and the preferred maximum being 0.1% = 1:1000, with India Railways even limiting the preferred maximum to 1/1200 = 0.083̅ %. Most metros limit the exceptional maximum gradient to 0.5% = 1:200 in parking and coupling/decoupling areas, with 0.3% = 1:333.3̅ being the normal maximum and the preferred maximum being 0.15% = 1:666.6̅ or 1:660 = 0.1̅5̅ %, which are almost identical to each other. Most light rails and monorails limit the exceptional maximum gradient to 0.5% = 1:200 in parking and coupling/decoupling areas, with 0.3% = 1:333.3̅ being the normal maximum and the preferred maximum being 0.25% = 1:400.

Obviously, trains have been known to run away at exactly a 1% grade, with the Federal Express train collision in Washington Union Station on the Pennsylvania Railroad (now defunct) in 1953 happening with its brake failure on the approach with a grade as little as -0.73% (≈ -1:137 gradient), so that isn't a gradient that will prevent a train from rolling away. So, will a 0.5% grade prevent an unpowered, unbraked train from beginning to roll in the absence of external forces? Also, Pennsylvania Station in New York has a constant grade of +/- 0.4% = 1:250 on either side of the change in grade at the center of the station. So, for curiousity purposes, if any type of train or railcar (even those that do not operate in those tracks such as MTA subway or NJTransit River Line light rail vehicles) is parked entirely on either side of the change in grade, will that be gentle enough to prevent the same train from beginning to roll into the tunnels under the rivers? How about 0.35%, or even 0.3%?
 




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