The affected passengers were issued a printed receipt by the on-board ticket inspector clearly showing the 100 ThB as 'Fee'. This is a policy that has been in place for decades to deter pax attempting to evade ticket payment. Pax are required to buy a ticket prior to boarding and present it for inspection on demand.
The problem here is a the station which closed its ticket office too soon or, more likely, too many pax turning up to buy tickets when train arrival was imminent.
I would not label this event as corruption.