It’s all about the arches. When it opened in 1904, the City Hall train stop stood as the elegant centerpiece of the Interborough Rapid Transit Company’s Manhattan Main Line. Arch-happy architect Rafael Guastavino whipped up a graceful beauty: The train platform runs beneath a curving ceiling striated by great ribs of tiled arches. During the day, skylights honeyed the tunnel below, augmenting the light cast by a series of bronze chandeliers. It’s hard not to think of a snake as you survey the tight coil of track or the lobby’s gentle, even sensual curves — the station, which closed in 1945, feels like some grand jeweled husk left by a serpent that grew far beyond it. In order to tour the station, you must be a member of the New York Transit Museum (an investment of $50 — $65 for a dual membership; $35 for students and seniors) and spring for a $50 ticket, but the thrill of haunting this dead space — and getting the lowdown from the museum’s experts — more than justifies the price tag.