Let's calculate all the different options :
First, the travel time if the Jumbo Jupiter can do the trip at full speed : 620km at 300km/h takes 2hours and 4 minutes to complete.
Option 1 : A four hour delay would mean they miss their train in Hicksburg (4h + travel time > 6hours)
>> no point in going for option 1, better to simply cancel than perform the emergency fix to no avail.
Now let's investigate option 3 : 90min delay + travel time >> they would arrive before midnight and would get their connection. The price of the new motor is 12'000$, much less than the 30'000 needed if they miss their connecting train.
Finally, option 4 : 40% of 300 km/h = 120 km/h. It would now take 5hours and 10min to do the trip. This would mean the passengers would arrive at 11:40pm, 20min before their connection departs. This also doesn't cost anything to the company.
Therefore, option 4 is the best option. People would get their connection, the company wouldn't have to pay anything, and could even use part of the economy (12'000$ for the motor) to give some compensation to the passengers (a drink or sandwich in the train for example).