Either this site is starting to fall apart or I'm being targeted, as I've submitted two similar replies to this thread, only to have them both disappear.
If you can't get the shop initials to match that particular car, use different ones: some suitable for NYC would be AJ, AO, AV, B, BC, BG, DE, EB, EC, ER, RT, UR, and WD.
Besides those, you could also use shop symbols from any common carrier railroad in North America, as it was regular practice for railroads to do re-weigh and re-pack service on non-homeroad cars which showed up on their tracks at the approximate time when such service was required.
At the end of each month, each road would send bills to those roads for whom they had done such work. This was done continent-wide, with universal set-fees, and ensured that the majority of the rolling stock in North America was properly maintained whether it was on home rails or not.
On my own layout, I might do the majority of cars with data specific to the cars' owner, but always include a few which represent other roads' shops, including those of my freelanced roads. And likewise for those freelanced cars: they might get re-weighed or re-packed by a real railroad, too. This helps to demonstrate the concept of interchange, where cars might roam the continent.
Edit: Well it's nice to see at least one of my earlier replies managed to show up, too, so my apologies for the sorta duplicate responses.
Wayne