In addition to the military's rule of 6Ps (Perfect Planning Prevents Piss-Poor Performance) I add six more of my own for recording in circumstances like this: Permission, Politeness, Persistence, Patience, Publicity & Payment. Basically, always ask permission first. At best, it gets you a bit of preferential treatment; at worst, it prevents you from being shot. See the Pinter link at the end of this.
Most station concourses are not, strictly speaking, public places, they are privately managed areas. (In the UK, all railway stations are owned by Network Rail and, with the exception of the busiest stations which are also operated by Network Rail, managed by the primary train companies that operate from them.) There will be a station manager who, unless you're using stealth equipment, you should contact before you undertake a recording and he/she may allow you to make recordings on a verbal agreement, or may require you to apply in writing. In the case of London Underground, for example, you need to apply in advance, fill out a form and pay a fee. The use of tripods, stands, etc., needs special permission in most cases and is not recommended at peak times. If someone does trip over your kit, you'd better have good public liability insurance, especially in today's over-litigious society.
The smaller and more unobtrusive the kit is, the less likely you are to be bothered by the "hello mum" brigade, but it really is part of the job to deal with this.
I've written about some of my experiences for various magazines and lectures: Searching For The Perfect Wave: A Life Recording Sound Effects | Once upon a time, I was involved in a small and vastly underfunded theatre company's production... | Sept 2008 | Theatre content from Live Design Magazine “Who’s Breathing?” Asked Director Harold Pinter | Theatre content from Live Design Magazine Writing