Im sure the company will later invest in a second train or maybe already has but is yet to arrive, but there is a lot more than just the initial costs. Trains have to under go repairs they also need to under go complete re builds after a certain amount of cycles or if it doesn't reach that it will be a certain time frame.
The thing about the set up at Arkham is its very much like Jet rescue, If the ride is staffed correctly with one train you can still on most decent attendance days push a larger capacity then the manufacturers estimated one, that and keep the attraction running smooth. For rides like Jet Rescue and Arkham which would in most cases run with one train unless its a high peak period, the second train really only acts as a convenience when it comes to annual maintenance, Instead of downing the attraction for a long period of time to conduct a re build it can happen while the second train is in operation.