Thanks Jp for translating this page.
In a railway station located somewhere between Paris and Lyon.
In this station, the frequency of the trains in the Lyon-Paris direction and Paris-Lyon are the same : 1 per hour. It remains constant in time.
A person who would randomly go on the gate (anytime) and wait for the first train to pass, whould either have a chance to see the Paris-Lyon or the Lyon-Paris.
The following month the chances to see the Paris-Lyon in first falls down to 1 out of 12.
We ask you to find what changed, knowing that the trains frequency remains the same.
Congratulations XBdE on his solution of the 09/22/02 but don't look at it, it's always better to find oneself...