Maria has two boyfriends, Juan and José. To visit Juan, you must take the train northbound and to visit José you must take the train southbound. Both trains pass every 10 minutes and as Maria likes both boys equally, it is not fixed if a train goes north or south and goes up to the first one that passes.
However, for some reason Maria ends up visiting Juan 90% of the time and José only the remaining 10%.
The reason is that the train that goes south passes 1 minute after the train that goes north. The only way to take the train south is to arrive at the station by chance in the minute after the train goes north.
If you arrive at any other time, you will take the train to the north that will pass first. For example, if the train to the north passes at 8:00, 8: 10, ... and the train to the south passes at 8:01, 8:11, 8 ..., if Maria arrives at the station at any time between 8:01 and 8:10 will take the train north. Only if you arrive between 8:00 and 8:01 will you take the train from the south.