Police car in front of the Leiden Central railway station

The unsuccessful criminals who have to travel by train often have the habit of not paying their fare. If that happens and the culprit doesn't want to pay the fine or show his ID, the train has to be stopped at a station and the police has to be called. This procedure is rather cumbersome, certainly if you consider that the police doesn't always show up in reasonable time, so many train guards don't check tickets anymore. Much easier than creating a lot of fuss.
There could be a different explanation for the police car: the officers could be buying lunch for instance.