There's a taxi rank there at the Amtrak station. That's probably the best option. They're all hoping for runs into DC, though, so I would give the cabbie $20 to make the quick trip to the rental car center worthwhile.

Plan B would be the two-free-shuttle trick into BWI and out again to the rental center. That could take 18-20 minutes if you hit it right, 40 minutes if you just miss buses.

Plan C would be Uber but I can't estimate wait time or cost.

I expect none of the central-facility car rental companies will come fetch you because none have their own dedicated shuttles anymore.