Hi,
On 125 mph mainlines there should be 10 feet between up & down lines, so unless you are 10' & 1/4 in height you should be OK!! The real problem is on the off side where some lineside structures, bridges and tunnels, even trees and branches may not be as far as you think. On lines below 125 tracks may be just 6' foot apart and at large stations complexes even closer.
However, saying that, I repeat the question already asked "Why would anyone want to lean out so far on a moving train"? You would need to be brain dead, or a moronic head-banger or something.
I once had three mindless morons climb on top of my DMU between Slough and Ivor. They would have gone farther had not someone on an overbridge point at my train as I approached. I couldn't believe when looking back from the cab, to see three youngsters on the centre coach roof. These sort of brainless idot are the reason why we must never reduce the driving age any further, for cars or trains. Even when confronted they couldn't remove the smirk from their faces as they were just too mindless to know the danger they were in and the affact they would have had on me, as the driver, if they had been serious hurt or killed.
If anyone reading this thinks that was a 'Cool' thing to do, you want to try attending a Coroner's court when as the driver you have to face the parents who have just sat through listening to what happened to their child and what the injuries were. And that goes for suicides too.
Enjoy your hobby by all means but don't go doing anything stupid.
Cheers.
BillEWS.