Trains that normally carry 242 passengers had 450 booked for the two hour trip from Gävle to Stockholm.
The fault in SJ's booking system was discovered and repaired in June, but by that time many trains had already been over-booked. A summer of chaos for passengers is forecast with over-full trains and replacement buses across Sweden, with Gävle the worst hit area.
"We are trying to keep a check on all the major events where we can predict that many will choose to take the train, then we can stand ready with replacement buses. It is regrettable, but there is nothing else we can do," said Ann-Sofie Bäck for SJ to Arbetarbladet.
110,000 people travel with SJ each day from 220 stations in Sweden. The summer is the busiest season for the rail operator as the Swedes take their holidays.