Looks like Amtrak offers 10-trip and monthly tickets on this route. If you know exactly which trains you'll be using, you can also save money by buying 14-days in advance. If you specifically want to use Amtrak, you may get better advice over in the Amtrak forum. A monthly pass on Amtrak is kind of spendy, but depending on your job it might still make sense.

Depending on where you live in the Baltimore area, you may find it tolerable to drive to Newark, DE and take the SEPTA commuter train. You'd pay I-95 tolls but save a ton on railfare.