depends on how confident and proficient you are at it. to start i suggest getting on the rail a little more than perpendicular (just a little, so your tips are kinda pointing back at the in run) and as you fall off the rail spot your landing and it will come around. depending on how far the landing is from the rail you might need to load with your arms and/or pop off the end of the rail
"In rode the Lord of the Nazgul. A great black shape against the fires beyond, he loomed up, grown to a vast menace of despair. In rode the Lord of the Nazgul, under the archway that no enemy ever yet had passed, and all fled before his face.
All save one. There, waiting silent and still in the space before the gate, sat Gandalf upon Shadowfax : Shadowfax, who alone among the free horses of the earth endured the terror, unmoving, steadfast as a graven image in Rath Dinen."