I'm sure folks dropped rubbish (papers, fag ends) in the 50's, though probably fewer than now. And there are lots more people now - the world population has doubled twice since WW1 and I guess the UK may be in the same range.
There would not have been the plastic waste, bottles, crisp packets, sweet wrappers, that stay so bright and visible today - newspaper would fall apart in a couple of rain showers, fag ends were probably not plastic either in those days, and similarly would degrade quite quickly.
I'm also pretty sure that there were station staff who swept it up on an hourly or daily, rather than quarterly or annual, basis.