To give a real explanation one would have to repeat the experiment under controlled conditions, which is not simple with a moving metro cab.
Steel objects will move and be attached to magnetic fields: think of the magnetized scissors picking up fallen pins.
If it is an electric train, which a metro would be, there are strong currents converted for use for the motion of the cab. Secondary currents could be induced in the metal body of the floor, or metal components beneath it, variable depending on the strength and position of the cab under the power lines. Currents are accompanied by magnetic fields. This may show the behavior observed, as with pins and a nearing or further away scissors.
It may be a motor that goes on and off underneath the car, for some reason, braking or increasing/decreasing in energy .
All this is handwaving guess work. Here is a link describing how electric trains are powered, and it will all depend on the exact system and the exact line and the exact geometry of the cab,