40 min service would require 2 trainset + crew operating, there would be long layovers at each end (not necessarily a bad thing, as it's nice to find the train waiting for you at the terminal rather than having to guess if there's service. 30-40 min service would reduce layover times). 60 minute service can be done with one trainset + crew.
Some quick back of envelope calculations, (I'm by no means an expert at estimating passenger demand). Assuming 40% the 13,500 are business travelers and or originating from the dt core and 15% opt to take the UPx if available. Gives 1,013 potential riders, add in say 5% for employees gets us to 1,064 potential passengers. The two car trainsets hold something like 115 passengers, with 40 min service we would yield ridership of 54 per trip, or with 60 min 89 passengers per trip. Maybe my estimates are best case scenario, I don't know. But a passenger load factor of 46% seems pretty good.
I think the issue is the availability of trainsets and being able to run the service while still being able to pull trainsets out for routine maintenance.