How Close is too close?

Whilst out at Newhaven i had to make use of this foot crossing at Tide Mill (so named after the old Tide operated Grain Mills that have long since gone).
I was wondering how close i could get to the train and still get a picture without distressing the driver or joining the statistics.
This gate was as close as i dare get - and only then because the gate would stop me being sucked under the train!
In case you're wondering - less than 3ft!