This is the way I used to get lots of credits inside of Terminal Railways. If you have a better way, please let me now in the comments so I can make an updated version.
Correction: You do not need to wait for the passengers numbers to start moving, they've already loaded. All it's doing is counting them up. You can even use credits to lower the onboarding time so you can wait less to start moving again. Hope this helps :)
I farm credit from dinsmark central. Lots of passengers spawn for northwell, linshire, rozlyn. You just have to change the route just before linshire station to get to rozlyn quick. The other routes you have to figure out yourself.