That's every Southern train. It's quite common to have to disregard several seats before daring to sit down because they're filthy.
I'm not saying it never ever happens, but in the last three weeks, I've travelled on public transport in Amsterdam, Leiden, Frankfurt, Heidelberg, and Paris and I've not seen anyone with their feet on the seats opposite. Mind you, it's a bit of shock to see clean streets (admittedly, the Parisians would be up to their necks in the shit of little dogs if they didn't).
Back in London yesterday, getting on a Thameslink at St Pancras, the first thing I saw. So I told him not to. Which he actually did and sat there sulking all the way to East Croydon before giving me a foul look as he got off. He must have been in his thirties, ffs.