London St Pancras railway station

Where it sits. St Pancras stands on Euston Road between King's Cross and the British Library, a red-brick Gothic cliff that rises straight off the pavement and stops most people walking past it. It has been the London terminus of the Midland Railway since 1868 and, since November 2007, the terminus for Eurostar services through the Channel Tunnel as well as domestic high-speed trains to Kent, East Midlands services to Sheffield, and Thameslink trains running north to south beneath the building.

The engineering. William Henry Barlow's train shed of 1868 is the piece to look at first. Its single wrought-iron span of roughly 240 feet, rising about 100 feet at the crown, was the largest enclosed space in the world when it opened, and it works because the deck acts as a tie holding the feet of the arches together. The deck itself sits on a forest of cast-iron columns spaced to a grid derived from the dimensions of a beer barrel, because the undercroft was designed as a bonded warehouse for ale brought down from Burton upon Trent. That undercroft is now the shopping arcade and the Eurostar departure hall, which is why the ceiling height there feels so oddly domestic beneath such a colossal roof.

The hotel. The Gothic frontage is not the station but the Midland Grand Hotel, designed by George Gilbert Scott and opened in stages from 1873. It is built of hard red brick from Nottingham with Ancaster stone dressings and Butterley ironwork, with a clock tower rising about 250 feet and a grand staircase that spirals up through the west wing. The hotel closed in 1935 and the building spent decades as railway offices, dilapidated and repeatedly threatened with demolition until a campaign in which John Betjeman was the loudest voice secured Grade I listing in 1967. It reopened as the St Pancras Renaissance Hotel in 2011. Inside the station you will find Martin Jennings's statue of Betjeman, hat in hand, looking up at the roof he saved, and Paul Day's nine-metre bronze of an embracing couple, The Meeting Place, at the south end of the upper deck.

On screen. This is the building that plays King's Cross in Harry Potter and the Chamber of Secrets. When Harry and Ron find the barrier sealed and take Arthur Weasley's Ford Anglia, the exterior they burst out of and the facade the car climbs past is St Pancras, not King's Cross. The substitution was pragmatic: in 2002 the real King's Cross frontage was still hidden behind its low 1970s concourse and looked like nothing at all on camera, while Scott's turrets and pinnacles a hundred metres west gave the shot a skyline. The interiors of the platform scenes were shot next door at King's Cross on platforms 4 and 5. It is worth knowing this because the two stations are constantly confused: the arrival and departure sequences are King's Cross, the Gothic exterior is St Pancras, and Platform 9¾ is at neither of the platforms it is named after.

What you actually see today. Walk in from Euston Road and take the escalator to the upper deck, where the platforms sit at first-floor level so that trains could cross the Regent's Canal north of the station. The Barlow shed's ironwork is painted sky blue, a colour chosen during the restoration after paint archaeology on the original scheme, and it is at its best in the late afternoon when western light comes through the end screen. Along platform 1 runs Searcys, usually described as the longest champagne bar in Europe. Two public pianos, one of them donated by Elton John, sit near the Grand Terrace and are almost always in use. The Arcade in the undercroft holds shops and a food hall; the northern extension of 2007, glazed and modern, is where Eurostar trains actually stand.

Getting there and timing. King's Cross St Pancras Underground station lies directly beneath, on six lines, and Thameslink runs deep below that. The station is step-free throughout with lifts between levels. It is busiest between seven and nine in the morning and around five in the evening, and photographers generally do better mid-morning or after nine at night, when the shed empties and the roof lighting takes over. Passport control for Eurostar closes well before departure, so allow at least an hour if you are travelling.

Nearby. The British Library sits immediately west, with the Magna Carta and the Sir John Ritblat gallery free to enter. Behind the station the Regent's Canal, Granary Square and Coal Drops Yard are ten minutes on foot, and the old St Pancras Old Church and its churchyard, with the Hardy Tree's former site, are a similar walk north.