To the Original Poster,
No, just No, everything about your question, No.
Lets say you had jumped down to retrieve you phone. Lets just say you had managed to get back on the platform safely before the train came in. Lets just say that you hadn't been seen doing so, and you had safely carried on the rest of your life in the knowledge that you had weighed up the risks that day and had retrieved your phone. THERE IS A 3 YEAR OLD CHILD STANDING RIGHT NEXT TO YOU. Not least, IT IS YOUR 3 YEAR OLD CHILD. (Yes I do think the capitals are nessesary)
How about:
Instead of it being Platform 8 at Doncaster, "where all trains stop and none ever seem to go at more than about 10mph", it was at one of the many platforms across the country where express trains fly through at almost full speed. Ever seen the Mail Train fly through the platform at Wigan, or the none stop Manchester to Liverpool fly through the platform at Huyton?
Instead of it being Platform 8 at Doncaster, where "there is room on the other side of the line to walk if one has to" it was one of the many platforms in the country where the track on the other side is so close that when an express passes through, all of the windows of a stationary train next to it slam shut.
But even after all that its ok 'only an imbecilic idiot would actually jump down onto the tracks as a train approached'....
Or a 3 year old boy who instead of retrieving a £200 iPhone was trying to retrieve a bit of rubbish that had caught his attention, who once saw Daddy do it so it must be ok. But little 3 year old boy is not as big as daddy and cannot lift himself back on to the platform. Oh and little boy didn't go travelling with Daddy today, who may have been strong enough to lift Justin Jnr back onto the platform, he went out with Mummy, or Grandmummy who isn't. Oh and Justin Jnr didn't realise Daddy had been observing how the trains operate and had checked for one first.
As for comparrison with pedestian level crossings where you walk straight across from one side to the other, usually on a special surface to reduce the risk of tripping or slipping.
Or comparrisons with road running trams in Sheffield where plafrom height is 6 inches or so (Yes I have used them), the comparrisons are totally irrelevent. Yes I have seen on numerous occasions people jump down from the platform run across a track and jump up on the otherside, all to avoid the 30 steps up and down to use the provided footbridge. I save a few choice phrases to mutter under my breath, for people who do that, none of them are nice and none of them are suitable for publishing here. The fact that the thought even crossed your mind to demonstrate that level of stupidity in front of any impressionable youngster is beyond contempt, never mind the fact that as I pointed out at the top, this is your child we are talking about. Yours. I don't have any children, but I will tell you what, if I found out my partner had done that in front of my child, it would be seriously considering if they where a fit and proper person to be sharing custody of my child. But frankly if I had seen you do that in front of your 3 year old child, it wouldn't be the BTP I would be calling, it would be the NSPCC. You may think that is a bit strong, but I don't, and I don't particularly care what you or anyone else on this forum thinks of me for saying it.