Work out the transport question before you need it, because the day you need it is usually the day something is wrong. A dog that cannot get on the subway travels by car service, and rideshare pet policies are at the driver's discretion, meaning a refused pickup on a cold night is a normal occurrence rather than bad luck. Dedicated pet transport exists in this city and is priced accordingly.
The practical answer most owners of this size of dog arrive at is a well-made soft carrier bought early and introduced properly, long before a stressful journey. A dog that has slept in a carrier at home a few dozen times gets into it on a bad morning. A dog meeting one for the first time in a corridor at eight in the morning does not. The carrier is a modest one-time purchase that converts an unreliable journey into a reliable one.
The joint side deserves a mention. Patellar Luxation is documented in this breed, and a small dog repeatedly jumping down from a bag, a bench or the last three subway steps is loading the joint in exactly the way that provokes it. Lifting the dog down rather than letting it jump, on the stairs and out of the carrier, costs nothing and directly addresses the mechanism. Choose the clinic you can walk to for routine work, and know in advance which emergency hospital you would head for and how you would get there.