I'd say St Pancras Intl, on the 14th most of the upstairs will be opened up including the champagne bar. I *think* that the area around the buffers (obviously behind glass) might be open so you'll be able to see them come in and out. I guess you'll have to go there and see! Have a look round the station..it's impressive! Alongside the champagne bar and up to Midland Mainline..sorry..."East Midlands Trains" as it'll be known, there's a think piece of see-through glass between you and the platforms, so if there's a train on platform 5..it's right infront of you..you can see how large the trains actually are.
anyway..just have a look around the station, you could be lucky with what you see