Step one: PREVENTION!!!!! Stand the heck back from the edge, FCOL.
If the ties go all the way across (there is no drainage trough) YOU WILL NOT FIT.
If you cannot see a large space that is under the platform, then don't go there. A lot of these openings have been fenced off as well, so look out. Rats and homeless people live down there, so hope you get along with rodents and schizophrenics.
I would never, ever, try the "corner" on the non-3rd-rail side. Death. You'll have no way to gauge that one until its way too late.
Running on the tracks, as suggested above, is much much harder than running on a flat sidewalk. There are innumerable things to trip on, and then there's the ever-present subway sludge which is a tremendous slipping hazard. WALK carefully IF you have the time.
Clearance niches work great, and as long as you stay calm, stepping over the third rail to get there is not so problematic. The wooden protection board is wider than the rail is. Between columns works too, just watch the traffic on the adjacent track. Stand sideways and center you shoulders between the columns.
Don't assume that trains can stop shorter than their car stop marker. Proficient train operators often take a full brake application in the middle of the station and stop right on the mark, but there's no headroom there. (Full service braking is air brakes AND dynamic brakes, emergency braking is only air brakes. The train is not practically going to stop any sooner from 30mph if the operator lets go of the handle and dumps the train.)
Again, please, please observe step one above.