I take a train with on-train checks about twice per year. I've seen people sneak past, personally, several times in the past ten years, which implies that they're harder to carry out than if people have to walk through a staffed gate.

Ths UK has this minister who seems to really really really really really not want people sneaking into the UK, not even if it's a handful per year.

The two paragraphs could be causally connected. I don't know.