Traveling Toddlers in a Taxi

Does anyone have any experience traveling in cabs with small children?

I have a friend that is going to NYC this weekend by train with his family and will most likely have to take cabs while in the city.

His son is about 15 months old and of course would normally be in a car seat. Ideally he would bring the car seat along but if he could avoid it, that would be his preference.

Any suggestions would be appreciated!