The Railway Museum smelled of dust, grease and a history that never sleeps. Nicolas, a seventeen-year-old volunteer, had been able to move around here almost in the dark since he was a child. That evening he brought a torch, a notebook and a stubborn conviction that he would find something unusual. He stopped at the huge clock from the station hall, which ticked as if out of rhythm. Something else could be heard in the quiet ticking, a low murmur, rather impossible at night. Lena came in late, with a USB cable in her pocket and a laugh to stifle her trepidation. Under the display case, they found a metal case labelled with an inventory number, without any key. The lock had let go under the pressure of the clips, and inside lay something like a pocket astrolabe. The brass disc had glass, two movable rings and a thin needle, quivering like a compass. Underneath it was a letter signed: Dr Helena Rutkowska, Department of Motion Synchronisation, 1973. The instructions were brief: synchronise to the sound source, enter the date, keep to the tracks. The tracks, they thought simultaneously, looking at the cast-iron rails cutting through the floor like a river. Nikolai set the first ring to today's date and the second to zero hour. Lena applied the device to the clock case, counting the ticks until she hit the perfect beat. As the needle trembled harder, photographs flashed in the display case behind the glass, as if someone was blinking. From the loudspeakers, long dead, came a station announcement about a train that had not been running for a long time. The voice said the date: sixth of June two thousand and forty, platform three, delay two minutes. - Is this a joke? - Lena glanced at Nikolai, but his face was unnaturally pale. The clock chimed, the mechanism inside working like a locomotive, although no one was winding it. The louvres of the skylights vibrated, and a strip of light appeared between the rails, narrow as a blade. A brass disc began to make a sound, exactly like a whistle we only know from the movies. On a piece of paper that had not been there before, the words emerged: don't go in together, don't come back this way. Beyond the strip of light, the image of the hall blurred, giving way to a platform bathed in afternoon sunlight. The year forty was flickering on the blackboard, and people had backpacks made of a material he didn't know. Nikolai smelled warm metal and ozone, and his phone lost its range. Someone on that platform raised his head and looked straight at them, as if he could see them. - 'Santa, don't be late,' said that someone, in his voice, only a few years older. The strip of light suddenly widened like a door opening, and the needle stopped shaking. Nikolai lifted his foot over the rail, feeling that one decision would rearrange all the turnouts.