I just rented from here last weekend.
No Aisle service but they had expedited counter service. The lady already had a contract printed and a car ready. 2013 Avalon with 3300 miles on it. (PCAR)
Amtrak's doing LOTS of construction so the cars are in a new outside lot off 29th and Arch. They even moved the Zipcars and Enterprise CarShare vehicles to that lot. It's about an 8 minute walk from the counter in the main terminal.
The car I had was from a spot that was labeled "Emerald Club"...others in the section were the usual Chargers, Impalas, etc.
Return was easy...once you enter, you told the greeter you were returning and he directed you to a spot to pull into. They have the handheld computers so he was able to punch in mileage and gas. A receipt came out and I was on my way.
All in all, a pretty good experience...just allow yourself some extra time if you're trying to catch a train or somethin'...